lots and lots of research(on most of the skimmers on the market), id did when i was thinking about getting one for my 75(this tank is about 7 months young now), talking to those who have skimmers, and reading everything i could find on them, and no not advertisements either, i also do know lots of people with them, and such(i did say, IMO and not IME)
i also keep curent on them too, b/c i am trying to make it a skimmerless system(lucky so far) but am ready to get one if need be too, just so we do not have any problems, i am all for them and not against i do believe tehy are one of the best tools money can buy for an aquarium(but we wanted teh challenge)
for the record, in a smaller tank, for under 100$, the only 2 i would consider are
the prizm and the seaclones, personally