But I want occasional enemies that are challenging. I'm not doing dungeon crawls and treasure hauls, but occasionally the PCs will have to deal with one very significant creature that raids the country or lives in the deepest catacombs under a castle. I want to be able to tell the players if it looks like a fight they can win if they are careful, or if the only option is to run and prepare a trap that keeps them way out of reach of any of the creatures attacks.
Hmmm... I'm taking it you are a first time retro gamer?
I'd never really thought about how hard this could be if you were accustomed to 'new style' play.
First, it's really not that old school to give players direct meta-information about the fights that they are facing. You can and should give hints that the fight is difficult through color, flavor, and setting, but outright telling them how to face a threat and how much threat it represents seems to be an attempt at making it all a game (which, I'm coming to see is how you view RPGs) rather than a simulation or narrative (or both).
You can suggest a fight is hard by...
a) Emphasizing the great size of the monster. "The head of the dragon towers above the surrounding trees."
b) Comparing the monster to what they've faced before. "You've never before seen an ogre of such size. It's muscles bulge and its torso is as round as a barrel."
c) Dramaticly describing the setting, "As you open the door of the tomb, there is an outrushing hiss of air. The sky above suddenly becomes dark, and there is a crash of thunder that causes the ground to shake. A rushing squall brings stinging cold rain as the day turns into night."
d) Providing hints: "You manage to decipher the runes embedded in the wax seals that are placed on the door. They are a warning about what lies beyond such as what might be used to seal a plague tomb. However, the chief symbol is not actually that of pestilence. The chief symbol of them is one that means 'death', particularly death associated with the use of necromantic magic. It indicates that what is behind the door is not a disease hazard, but one of dark magic. The rune is annotated by the number 5, which indicates the level or degree of hazard represented. In this case, the five indicates the possibility of death to all life in a widespread region, such as an empire or a continent."
e) By gradual transition to grander surroundings, "The fifth staircase brings you to a sort of narthex of a temple larger and grander than those that have come before. The ceiling is lost in the darkness above, and the great columns that run in a double row to either side are made from a black stone flecked with red and carved with writhing serpants."
f) By emphasizing how completely outclassed they are, "The woods around you are suddenly filled with howls as from a host of wargs. In the twilight, you can see perhaps 60 yards through the boles of the forest. In the gathering darkness, you can see dozens or scores of goblins riding loping wargs in ragged formation to your north and west. The are moving roughly to the south-west. Many bear flags streaming behind them, showing the devices and heraldry of at least a dozen clans. Bats are streaming through the forest over there heads. From the thunder of the howling wolves, the singing goblins, and the crashing of spears against shields, you feel certain that this is only the edge or vangaurd of a mighty host. "
Markers like that tell the players that they should possibly prepare for the worst, or flee, or that parlay is a better approach than combat, or that they are about to get in over their heads. But you should try to avoid actually telling the players what to do or what strategy you expect them to employ. If you do that, you run the risk of playing the game from both sides of the screen rather than letting your players choose how they want to play. Avoid linear expectations of 'they should do this, followed by this, followed by this, thereby obtaining level X so that they can face this.'
Secondly, again, you'll need to know what the particular PC's you have can face. Level alone won't tell you much. Eight 8th level PC's with treasure obtained from published adventure paths of some sort intended to force rapid leveling will be challenged by very different encounters and deem different monsters 'serious' than 4 8th level PC's with gear that has been randomly generated or carefully selected as to maintain a low pace of advancement and power acquisition. You need to know what your party is capable of. A party without a cleric is going to find wraiths and shadows very much more challenging than one that has a cleric capable of destroying shadows. A a party that has acquired skillful play skills is going to be very much more successful than one that only has straight foward close and attack strategies while dumping offensive spells in their tool bag. By the time you are DMing for a party of a given level, you should have a fair idea how much pain that they can bring per round on a typical foe with 4 AC, and conversely how much pain they'll recieve facing say 12 bugbears. You'll need to scale from what you know.