Welcome to the site! You will have fun on Blaster; I've owned both Blasters and a Warrior, and there is no comparison. Blasters put the Warrior to shame performance wise. They are also easier to work on and nowhere near as heavy. Ever gotten a Warrior to wheelie by only giving it throttle? Didn't think so. A Blaster will surprise you!
Yes, it is a good idea to check compression before buying a Blaster. Any reading below 120psi is a good sign that the top end is getting weak. Take a gauge with you and use it.
Start it, run/ride it, and look it over well. Be VERY wary of one that doesn't run. Most Blasters that don't run have bad top ends. If it passes a compression test and has lost fire or has carb problems, these things can usually be fixed fairly cheaply, but still. Buyer beware. Assuming it runs, how does the engine sound? Blasters (and 2-strokes in general) are noisy, but there shouldn't be any knocking or clattering coming from the engine.
What is the general appearance of the quad? Blasters are everywhere, and good deals are not hard to find, so it pays to move on if you are looking at one that seems beat-up or abused. Missing hardware, mis-matched plastics, lots of grease and grime, bald tires, rattling engines, brakes that don't work, more than a couple of these things all found on the same quad... These are some subtle signs that a Blaster has been run hard and/or abused. Move on.
You should be able to spot a machine which has been cared for. It usually looks fairly good, starts easily, and runs well. Some of the best-kept Blasters are those stock or near-stock machines which are being sold by the original owner or have only been used for trail riding.
If you can find a deal on a 2003 or newer Blaster, get it! You'll gain factory-equipped disc brakes, a huge plus. Hope that advice helps. As Blasterdad said, its not much different than buying a car...