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?
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.