the negaive lead is the shorter lead on any electronic componant of which corect polarities should be known, if you accidently snipped them both to the same size, then look for the highlighted area on the cap, its usually lighter but sometimes darker than the rest of the capacitors "wrapping" and often right above the neg. lead
look at this pic
EDIT: looks like texan beat me to it...o well atleast i got a pic right...lol