The law is recorded in Shulchan Aruch et al. that one who sees lit Chanuka lights and has not lit his own should say the b'racha of שעשה נסים לאבותנו ("...she'asa nisim la'avosenu..."). Could he say this b'racha on a light that was lit improperly (e.g. earlier than the allowed time)? Would his due diligence require him to find out whether the flame in question came about in the proper way?
The Beis Hab'chira (maseches Shabas 23a - "mi she'ein lo l'hadlik" on the lower left) quotes and approves of the opinion that if one has no access to Chanuka lights of his own and cannot see others' then he should say the b'racha of she'asa nisim... anyway, implying that seeing lit lights is a catalyst for the b'racha but that it essentially applies to the occasion and is bound more or less incidentally to the lighting. This would imply that one who lacked lights could certainly recite the b'racha upon seeing flames of uncertain legal origin because that would be no worse than seeing no flames at all.
מי שאין לו להדליק ואינו במקום שאפשר לו לראות יש אומרים שמברך לעצמו שעשה נסים. . . והדברים נראים
In other words, even though the Beis Hab'chira's argument can't apply directly to the case in question since it is one in which he actually does see flames, according to him, the implicit kal vachomer should at least suffice to overcome the concern of safek b'rachos to require/allow the b'racha to be recited in this case.
This Halacha only applies if you're not going to light your own Chanukiah later the same day, and you don't have someone lighting on your behalf.
You can only say the beracha on a light that is still within the required half-hour. If the light has been lit for longer than that, you cannot say the beracha. If you're unsure, you cannot say the beracha because of safek berachot l'hakel.
(Yalkut Yosef 676:7)
He doesn't specifically address your question, but I'm guessing (a) that one of the other reasons for not saying the beracha applies to you, and (b) that it's a safe extrapolation that you can't say the beracha on a candle that was lit too early. I'd guess, however, that you can generally assume the candles were lit within the proper time frame.