Well....my thoughts.
1. Does it have an ALT selection switch/button? If not, then there is no mode C, but having an ALT doesn't mean that the mode c is actually hooked up.
2. If it has the ALT selection switch/button, then there is often a plackard noting that Mode C info is correct up to 25000ft or something like that. Seeing that would imply that you do have mode C.
3. I don't know for sure, but I would suspect that if you have the ALT switch/button but the Mode C wasn't wired up, that there would be some sort of plackard to report it.
4. Ask the aircraft owner/operator (I'm assuming you don't own it.)
5. All else failing, fly it aircraft and contact a radar unit and ask if they are getting a mode C response from you.
...after that I'm out of suggestions!
dp