Unless you have the tools to locate it all you can do is look it over with the Mk I eyeballs. To really find and diagnose the leak you will need some specialized equipment and even then it is only so good. I recommend the use of UV dyes. Charge the system up, install the dye, wait for the a/c performance to fall off (indicating leakage) and then go over the entire system with a mirror and the special light and glasses that will cause the dye to glow. The S70's have been a lot better about leaking evaporators than the older 850's but that does not mean that they are immune. If it is a leaking evap they can be a royal pain in the ass to diagnose because most if not all of the tell tale signs of a leak are on the side of the evap that you cannnot see from inside the car.
Your other option would be to get a "sniffer". The electronic leak detectors or "sniffers" have improve a great deal over the last 10 years or so and may be the way to go. But again it is a specialized piece of equipment that will set you back a couple of hundred bucks. You may be better off taking it to someone who is familiar with Volvos and their a/c systems and pay them the money to find the leak. Remember though, should you opt to fix it yourself it could get dicey with the repair shop fast if it has other leaks or it does not get fixed right the first time. That is why it is best to let a repair shop diagnose AND fix the car. It will cost you more money but at least you will have a guarantee on the repair. I've seen it before where a tech diagnosed it and the customer went home and fixed it and for various reasons it blew up in everybodies face, kind of a case of too many cooks spoiling the soup.

Good Luck,
Mark