Remember that an IFR alternate is only required if the weather at the arrival airport is forecast to be lower than 2000' and 3SM within one hour before or after ETA. (Part 91 rules)

Some of the factors determining whether an alternate is legal to use include the forecast weather at the alternate (standard minimums are 600/2sm for airports that have a precision approach and 800/2 for non-precision approach), fuel required to travel to the alternate, and any restrictions that that particular alternate might have (as laid out in the A/FD and the approach plates).