The correct method as perscribed by Volvo is to add black light dye and recharge it, then (assuming no obvious detectable leaks are found) have the car checked with a black light when the freon has leaked out again as one will then see the dye, it's very obvious. Depending on how slow the leak is, it can take months to years before the freon escapes enough to once again cause AC cooling problems. The dye is evident anywhere it leaks (NOT just the engine compartment) so one can look at the evaporator and actually see it if and when freon has leaked, eliminating all guesswork.
The evaps cost about $250 plus freon but IMHO, it's not a job that most people w/o alot of automotive experience would want to try. It takes most experienced auto techs over 8 hours to do it the first time but they get better at it after doing it more often.