Piggy-backing chips often is worth a try - just stick the known good chip on top of the chip to be tested, such that all the pins are making contact correctly between the two. That way, if one chip has lost a connection internally, the good chip will set the output correctly as it'll be floating in the dead chip.
Testing for heat is not reliable. If it works, great, but the only time a dead chip will get hot is if there's a short circuit somewhere. If it's just a lost connection it'll be normal temperature.
The only surefire way to check for RAM being bad is to take the chip out and put it in a known good machine. One thing you can try though is to check the data line out of the RAM chip against ground - each of the chips should have roughly the same resistance between it and ground. If one is different, check that chip first.
