You can take a digital meter and put a lead on each
side of capacitor, the meter reading will go up and
then back down, Reverse the leads and it should do
the same thing.

If this is a dual capcitor that controls the fan and
compressor and only the fan comes on, more than
likely it is not the capacitor. However its easier to
replace the capcitor and prey to god that it works.

If it doesn`t then you know you have other issues for
sure, however the capacitor is a pretty cheap part.