Have it pressure tested as previously suggested. If there's a leak, a good tech will see it leaking or notice that it's loosing pressure and if it's loosing and the source isn't obvious, then he should first look inside by the heater core tubes and core itself, then do what's called a "block check". That's done with a device using blue liquid that'll turn yellow if carbon moxoxide's bubbling though it. It's placed over the coolant bottle with the engine running and helps diagnose bad head gaskets, very rare in Volvos.
One can go further yet by removing plugs while pressure testing and using a small telescopic flashlight or boroscope, see if coolant can be seen in any cylinders.
I have seen coolant in a cylinder that way once and found that the block itself was cracked in between 2 cylinders (rare occurance). It was under warranty and got a new engine.

Ditto on pablos' suggestion about the pressure test. You'll see where the coolant is leaking from right away.

My guess, is that it's seeping out where the plastic tank meets the aluminum core on the radiator (that's where mine leaked!) I added a "tube" of Aluma-Seal (About a buck and a half) and the leak went away.

Good Luck!
--
RussB in Ct. - '96 855T- '91 940T - '93 850GLT (R.I.P. 6-16-02)

can be done, no downside just need a blacklight

Why not pressure test the cooling system?
--
http://www.fidalgo.net/~brook4/oilslubesfilters.html