First off, to check if the thermostat is good all you need to do is remove it and drop it in a pot of boiling water. If it opens it's probably good (or at least good enough. If you have a thermometer thats even better. Just get the water to the right temp. Next, there are several ways to check for a blown head gasket (or cracked block/head) without tools.
Is the car running like crap, this is the sign of a blown head gasket. Is the oil or water a milky coffe color. If the car is running bad and the oil is milky this is also a sign of a blown gasket and or cracked head. If the water is milky and your getting excessive pressure in the coolant lines this means a cracked block. The problem is, any one of these can cause any and all of the above problems. Since you said it doesn't feel like there is any presure in the coolant lines, I don't think its the block (the only nightmare out of these problems) It sounds to me like your t-stat is bad, because if there is to much/not enough presure, your heater won't work.
Like I said, I don't think it's as bad as a craked block or even head. But to check for this remove the radiator car (overflow tank) then start the car up. If it bubbles alot and right away your getting exhaust gases into the ratiator system. When you do this look for smoke and smell for exhaust.
I'm not an expert, I've just had all of these problems on differnt cars at one time or another. Good Luck!
