Zimbel's suggestions are pretty well on track. The best way to decide if a new race is balanced or not is to compare a sample character of that race with a Core Rules character who has similar adjustments.
For instance, if you create a Small race with -2 Str, +4 Dex, -2 Con or whatever, you'd best compare it to a halfling, and compare them both as 1st-level rogues or 1st-level fighters (focused on Weapon Finesse, or the Point Blank Shot feat chain). If you created a Medium race with +2 Str, -2 Dex, +2 Con, -2 Int, -2 Wis, it'd probably be best to compare with a dwarf or half-orc, and make them both 1st-level fighters. It's also a good idea to compare mid-level versions, like 10th-level.
However, that comparison alone won't give you an obvious answer; one race or the other might have superior non-combat benefits, or superior spellcasting benefits, or whatever. Try and compare those other traits as well. Don't create a custom race stronger than the dwarf (unless you give it a Level Adjustment), and don't create a custom race weaker than the half-orc, half-elf, or human. Try for something equivalent to the halfling, elf, or human in overall effectiveness, after considering versatility (humans are good choices simply due to their versatility, and their bonus feat can be quite handy, even though a human is statistically weaker than several other core races).
As for ability scores, Strength is equal to Dexterity in importance, Constitution is equal to Intelligence, Wisdom is equal to Charisma. A bonus to Str or Dex is equal to a penalty on the other, or a penalty to both Cha and either Int, Con, or Wis. However, the race should get a minor compensation if they get -2 to two scores for +2 to just one; for instance, half-orcs really ought to get something like +3 on Intimidate checks to better balance out their Cha penalty alongside the Int penalty. A bonus to Con or Int is equal to a penalty on the other, or a penalty to Wis and Cha along with a minor compensation as noted above, but weaker since the disparity between +2 Con/Int with -2 Wis and -2 Cha is smaller than that between +2 Str/Dex with -2 Wis and Cha. A bonus to Wis or Cha is equal to a penalty on the other, or on Int or Con with a very minor compensation added.
Anyway..... I would suggest, take a gander at the various custom races on this forum, or at least a few of them, and see how they seem to be balanced. Some are certainly broken, but many are probably balanced just fine. Then you'll be better prepared to eyeball it later when you're making final revisions to any custom race of yours. I'd especially suggest taking a look at Xereq's race creation thread, it has several LA+0 races posted by folks.
For LA+0 races, try and avoid big special abilities such as regeneration, fast healing, damage reduction, flight, ability/level/energy drain, fear auras of any sort, gaze attacks, spell/power resistance, and such. Immunities to critical hits, sneak attacks, and some other stuff should probably be avoided or very carefully balanced. It's tough to balance a race of the Construct, Elemental, Plant, or Undead types, unless you take away several of the immunities and quirks imposed by those types. Avoid the Vermin type and probably the Ooze type as well, since they're normally supposed to be mindless and it'd be a stretch to give them Intelligence scores and whatnot; races vaguely similar to them in form could be designed as Aberrations or Monstrous Humanoids, though. Avoid the Animal type likewise. Remember that several spells and powers work on Humanoids but not other creature types, so belonging to a type other than Humanoid is generally at least a minor advantage.
Unlike other folks, I won't advise you against giving LA+0 races a bonus to Intelligence, Wisdom, or Charisma; you can balance it out with Dexterity and Constitution penalties, or similar drawbacks that make the race more vulnerable, or you can balance just the spellcasting/manifesting strength of such races with minor drawbacks that weaken their supernatural capabilities just a tad (such as a -1 racial penalty to the DC of spells they cast or powers they manifest).
I'm utterly befuddled by folks who think it's just fine to create races who are awesome at beating stuff to a pulp, or awesome at sneaking around and sneak attacking everything in sight, but throw a hairy conniption as soon as someone else even hints at giving a standard race +2 to a spellcasting stat, like Intelligence. People like me still play races in subpar roles despite such things; there are plenty of folks who like to play odd or challenging or fun combos like dwarven sorcerers or halfling barbarians or half-orc bards. So what if a roleplay-disliking powergamer always chooses the +2 Int race for his/her wizards? He/she's always going to choose the dwarf (or occasionally half-orc) for his/her fighters, after all. It's not like it's any different.
Here's some more specific advice/guidelines: I always try to get my custom races to possess between 3 and 8 feats worth of racial benefits after all drawbacks are considered, and after considering the value of versatility (if the race has any versatile features).
3-4 feats worth of racial benefits is fine if the race is highly focused on a particular archetype (such as melee brute, ranged attacker, durable spellcaster, specialized spellcaster, sneaky knave, cunning trickster, tough guy, socialite, wise guru, alert scout, or agile warrior). 7-8 feats worth of racial benefits may be fine if the race's traits are highly disparate and don't mesh well together, making them marginally good at several things but not very good at any one thing. Generally try for a race with 4-6 feats worth of racial benefits and a modest degree of focus to those traits, preferably with a bit of versatility or capacity to fill other niches.
Here are some feat-equivalencies I use, but circumstances may slightly alter them (frex, a Strength penalty is no big deal to a race who's great at spellcasting, but really hurts a race who's only good at fighting):
+2 Str = 4 to 4-1/2 feats, +4 Str = 9 to 9-1/2 feats, +2 Dex = 4 to 4-1/2 feats, +4 Dex = 9 to 9-1/2 feats, +2 Con = 3 to 3-1/2 feats, +4 Con = 7 to 7-1/2 feats, +2 Int = 3 to 3-1/2 feats, +4 Int = 7 to 7-1/2 feats, +2 Wis = 2-1/2 feats, +4 Wis = 5-1/2 feats, +2 Cha = 2 feats, +4 Cha = 4-1/2 to 5 feats,
human bonus feat = 1 feat, specific racial bonus feat = 1/2 to 3/4 of a feat depending on which feat it is, human extra skill points = 2 feats, low-light vision = 1 feat, each 40' of darkvision = 1 feat,
+1 on all saves = 1 feat, +1 on all saves against magic = 3/4 of a feat, +1 on all saves against psionics = 3/4 of a feat, +1 on all saves against poison = 1/4 of a feat, +1 on all Will saves against magic = 1/4 of a feat,
-2 Str = 3-1/2 feats of drawback or balancing +2 Dex, -4 Str = 7 feats of drawback or balancing +4 Dex, -2 Dex = 3-1/2 feats of drawback or balancing +2 Str, -4 Dex = 7 feats of drawback or balancing +4 Str, -2 Con = 3 to 3-1/2 feats of drawback or balancing +2 Int, -4 Con = 7 to 7-1/2 feats of drawback or balancing +4 Int, -2 Int = 2-1/2 feats of drawback or balancing +2 Con, -4 Int = 5 feats of drawback or balancing +4 Con, -2 Wis = 2-1/2 feats of drawback or balancing +2 Cha, -4 Wis = 5-1/2 feats of drawback or balancing +4 Cha, -2 Cha = 1-1/2 feats of drawback or balancing +2 Wis, -4 Cha = 3 feats pf drawback or balancing +4 Wis,
light sensitivity = 1 feat of drawback or balancing low-light vision, light blindness = 1-1/2 feats of drawback or balancing darkvision 40', severe light blindness/sensitivity = 2 feats of drawback or balancing darkvision of 60-80'.
You can check the recent thread about how much it takes for a race to deserve level adjustments to see a bit more discussion on part of this topic.
My Aurelia thread will gradually receive several racial descriptions and stats, and currently has them for just a few, but later it might be a good reference spot if you're curious how my balancing attempts work out. My Rhunaria campaign website already has basic info and full stats for each Rhunarian race, ranging from humans to dark elves to hobgoblins to kobolds and gnomes and more.