Yes, there certainly is. Most relays are laid out in similar fashion. They have a 'Coil' which is basically a small electro-magnet which has a feed and a ground. You might see a diagram on the side of the relay indicating which terminals are for the 'coil' (depicted by a 'looping' or 'spiraling' wire), just use your Ohm meter to make sure that there isn't an 'Open' circuit (there should be some resistance from one terminal to the other). The other part to a relay are the Contacts. most relays have a common contact that 'rests' on a terminal (normally closed), and then gets pulled onto another contact when the coil is energized (normally open). by refering to the diagram on the side of the relay, you should be able to check for continuity on the 'normally closed' circuit, and then by feeding 12V and Ground to the coil, you should have continuity on the other set of terminals (normally open).
The main causes for a relay to fail would be an 'open' in the coil (it's just a small wire looped around and around), when corrosion builds up on the contacts of the 'normally open' circuit (when the contacts close an arcing occurs), and lastly (seems to be popular with the "Volvo crowd") bad solder connections (which prevent any voltage or ground from getting through its intended path).
Please remember that the Cruise control 'turns off' when the brakes are applied. On most American cars, a feed from the cruise control circuit runs through a 'normally closed' ciruit in the brake switch (I think that it was typically a ground circuit for the coil in the relay). When the brakes are applied, the circuit opens and the cruise control 'cuts-out'. I am not sure how it is wired in a Volvo, but I remember seeing a bad brake light switch as a cuplrit for 'no cruise' more then once.
I'm having trouble locating the cruise control relay. Is this the relay that dangles down from four wires if you move some of the under-dash carpet panels? That is the only relay I could find, but I was expecting it to be connected to some kind of panel, not just some loose wires.