I have a wet gil system which stops approx 1' from the transom (hose connects SS pipe to SS the tips). Both manifolds and pipe are 9 years old in salt water. I bought the boat used so I don't know about flushing history. Are the manifolds reaching their life span? The pipes? Whats the best way to pressure test? Is it possible to test while installed (I dont think so but I'll ask anyway)? Would you even bother? Thanks for any input.

Some people will tell you to just change them. That it's not worth the risk. Maybe they are right, in salt water. I bought 2 salt water sets of Gils and have used them for 4 seasons in freshwater. I had one small leak that I repaired. To swap them now, they may still be worth $400-500 to some fresh water guys.
You can presure test by removing the manifold, makeup a fitting to hook a garden hose up to one fitting and a cap on the other. Turn onthe hose. The 40-50 psi out of the hose will find any leak that is there.