It would depend upon the high level character and the nature of the campaign world. It's quite possible that, absent disguises, PCs or NPCs might recognize high level characters who'd been in the area before by name--"You, priest of St. Cuthbert with a turtle shell shield and obsidian morning star! You must be Falon Kilbourne. You saved my cousin from the gnoll slavers last year!/you raised my sister's cousin's nephew's liege lord from the dead!" or "You, with the raven, silver glaive and the symbol of Pholtus; you must be Frederick of the Pale--weren't you on trial for killing those corrupt sheriffs in Almor two years ago?/didn't you face down the undead hordes on the streets of Midmeadow?/I heard that you (almost) single-handedly killed a frost-giant by Shorewatch castle." or "Hmm. Shiny fullplate, a greatsword, jade symbol of Pelor and the medal of Keiss Sor? Must be Diomedes Hopesoul--I heard he married into House Grek. (Unusual for a member of the nobility to marry a foreigner--even if he is Suel). So the woman wearing the platinum ring must be Thetis Grek. They're both skilled warriors; I heard they killed two powerful necromancers in the County of Urnst and he was awarded the medal for recovering the bodies of all those who fell in the defense of Shorewatch Castle. I hear the Boneheart put a price on his head after he killed one of their wizards."
For that kind of information, Knowledge (Local), Knowledge (Nobility and royalty), Bardic Lore, or Gather information would be the appropriate roll. Some individuals would have built up an accurate reputation, others would have exaggerated reputations, and still others might have no meaningful reputations as there were no witnesses to their exploits.
However, if they were disguised, and you were trying to guage their skill, it might be a great deal more difficult. I think a spot check would probably give information about apparent skill (Thetis is one of the most graceful people you've seen but Diomedes seems to have a fairly typical dexterity for instance). The difficulty of the spot check would depend upon the circumstances where they were observed. There's not too much difference between dex 18 and dex 10 when you're sitting down to eat dinner at an inn. If you're trying to make your way through a bustling crowd or dancing however, differences might well be apparent.
Aside from magic items (already discussed), a spot check combined with a spellcraft check might identify some material components of foci. If you catch a glimpse of a scrying mirror among the baggage tied to Frederick's horse, you might surmise that he can cast scry. Similarly, if you recognize that Diomedes and Thetis's rings are the foci for Shield Other, you might surmise that one of them can cast at least second level clerical spells. A higher level wizard's spell component pouches are probably a good deal more extensive than an apprentice's.
And if you see Frederick's raven having a chat with the grey thrush by the door, you can be pretty sure that Frederick has at least 7 levels of wizard.
As for telling base attack bonus, I think you'd need to actually see the characters do something that demonstrates their skills. If Bullseye is sitting in the tavern having a beer, you can't tell he's an incredible shot. OTOH, if he's playing darts, a spot check will tell you that the points of the darts in the bullseye all touch (so he can obviously hit AC 16-18 (whatever the bulleye is) without difficulty) and a sense motive will tell you that he's not even trying hard (so he can hit that number, even if he rolls rather poorly) Similarly, if you observe Robin Hood's bout with Little John, you can probably peg how many iterative attacks both of them have and whether or not they have TWF. But if you only observe them drinking in the greenwood, all you'll know is that Robin Hood won the golden arrow at Nottingham faire and that Little John beat him at quarterstaffs (if you've heard that).
Sense motive/spot would probably also be the mechanic for noticing whether the warrior in fullplate with the greatsword has worn the equipment long enough to be familiar with it and whether or not it looks used too. If the warrior keeps adjusting the baldric trying to find a comfortable fit or doesn't look used to the weight of his armor, he's probably green.
So, no transparency WRT levels. But you should be able to draw reasonable conclusions based upon equipment and observing class features in use. And characters may be appropriate subjects for Knowledge Local, Knowledge Nobility and Royalty, Bardic Lore, and/or Gather Information.