Go to a autoparts store and look for a a/ recharge kit, should
be about 25 bucks, It comes with a can of r134a, a pressure gauge
and the hardware to recharge your a/c, as far as knowing if it needs
to be recharged is based on the temp that comes out of the vents
and if it cycles a lot.

My 93 850 leaks r134a, I charge 3 times a year and it keeps me nice and
cool, the cans cost about 6 bucks I usally get two cans,

Pop the hood, the charge location should have a yellow cap with
r134a on it back left area of the engine compartment (READ THE DIRECTIONS)
on the kit,

a shop usally charges 100 bucks, not sure what they do for that
other than what the $25 kit can do, perhaps than drain the system
first.

My a/c has failed due to the horrible volvo design but the kit gets
me through the really hot months