make a test panel, see how it reacts. Send off a sample to a lab and have it tested. For aerospace this is required. The lab, usually the manufactuer's, will then give a re certification for 3-6 months.
If it's for non critical parts, do a test to be sure the resin flows and cures. I use old scrap stuff for my personal projects.
I'm assuming the prepreg was in the freezer the whole time? If not... then it's probably trash.