I think that it is a farily good idea to replace both on a regular basis. Let me offer this suggestion, get a new 'kit' to install. Before installation, trigger the one that is in there. This will answer several of your questions, and will also answer the other question on how good are the 'seals' on the vest overall.
If you really want to have a good test, put on the 'suit' and your PFD and jump in. Now you will not only have a good test, you will get some experience in the water when it fires up.
Unfortunately, I've averaged about one 'un-intentional' trigger about once every 18 months or so - so I've not had to check this or replace a canister/bobbin as a safety concern.
And while we are on the subject, I wear mine ALL THE TIME WHILE ON/IN THE BOAT. Everyone should.
Tex