If your Cannondale is an 86 SR800, it came with a Tange cromo threaded fork, according to the catalog. What size is your frame and how big is your head tube?

There are two things you want to consider with fork compatibility:
1. Rake. Check out the catalog for the rake specified for each size (it's listed in inches for some reason, so convert to mm). Not a huge deal if it doesn't match with your replacement fork but it's probably good to try to get it close or matching as rake affects handling within reason.
2. Steerer tube/thread length. The replacement fork will need to fit your head tube/headset. There is a Sheldon article on figuring out the exact right size and cutting it. Especially relevant if you're replacing with a used fork. I would have the bike shop do this part if you aren't familiar.