Only one way to find out:
Explore the system. The boat is 22 years old, so the odds that any original setup is still in place are virtually nil. It may have been changed several times by various owners by now. If there's y-valve or tee fitting in the tank pumpout line, and a hose coming off one side that goes to a manual or electric pump and then to a thru-hull, you have the capability to dump the tank. If it's not there, you don't. And while you're exploring, find ALL your seacocks, breakers etc and whatever they're connected to. An emergency isn't the time to START looking for them.