Although we generally recite the Berachot over the Chanukah candles specifically when we light the candles, there is one situation in which Halacha would require reciting a Beracha upon seeing Chanukah candles. Namely, if a person will not, for whatever reason, light the candles on a given night, such as if he is traveling and will not be in a position to light, and he does not have anyone lighting for him at home, then he recites a Beracha if he sees Chanukah candles burning. The Beracha he recites is "She'asa Nissim" the second Beracha one normally recites after lighting the Chanukah candles. The Beracha recited upon viewing Chanukah candles is generally referred to in Halachic literature as "Birkat Ha'ro'eh."
As mentioned, this obligation applies only if the person has not lit, has no intention of lighting that night, and has nobody lighting on his behalf at home. Additionally, one recites this Beracha only if he sees Chanukah candles within a half-hour of their lighting; after this point, the essential Mitzva has already been fulfilled, and one therefore does not recite a Beracha when viewing them. If the person cannot ascertain whether the candles have been lit within that time, he does not recite the Beracha, given the principle of "Safek Berachot Le'hakel" (one does not recite a Beracha if its Halachic requirement is uncertain). Furthermore, one does not recite Birkat Ha'ro'eh upon viewing an electric Menorah; this obligation applies only when one views proper Chanukah candles that fulfill the Mitzva.
According to one opinion, even if one has family members lighting for him at home, if he will not be personally lighting Chanukah candles on a given night he should make an effort to see somebody else's Chanukah candles. Even though in such a case he does not recite Birkat Ha'ro'eh, he should nevertheless endeavor to see Chanukah candles so that he can participate on some level in the Mitzva. The Peri Chadash (commentary to the Shulchan Aruch by Rabbi Chizkiya ben David DiSilo, Italy, 1659-1698) does not accept this opinion. Nevertheless, Chacham Ovadya Yosef (as recorded in "Torat Ha'mo'adim") advises following this view.
Summary: If a person cannot light Chanukah candles on one of the nights of Chanukah, and nobody lights on his behalf at home, he recites the Beracha of "She'asa Nissim" if he sees Halachically valid Chanukah candles burning within a half-hour of their lighting. If a person will not be lighting one but somebody will be lighting for him at home, he does not recite the Beracha upon seeing Chanukah candles, but he should nevertheless make an effort to see Chanukah candles burning so as to participate in the Mitzva on some level.
