Haha......... hiya Crafty.
Simple visual check for a start (ladder, so someone else)
First indication of trouble is "ponding" ........ puddles to you and me. Sure sign of potential problems.
Felt rotting as it drips into the gutter.
Common sense really.
Seriously, you'll need to trust someone who has made up these roofs for years. Ask in Builders' Merchants or Roofing Merchants for the teams that have been around forever. Cowboy roofers don't remain in business for a terribly long time usually.
They'll advise you with one quick glance.
There really isn't much, if anything, that can be patched in the interim. If it's on its way out, it will be stripped down to the deck, and replaced.
If ponding has led to leaks, then deck renewal is needed as well. (Much more money, so don't let it get to that stage.)
Well worth having someone trustworthy look at it if you have no idea of its age :o)))