Second Chance PCs

I happen to live near a few thrift stores that on occaision have older IBM compatible machines. Generally speaking, these machines are 386s and 486s. Can anyone recommend a good diskette based diagnostic tool or basic DOS command(s) that I can run, to check and see if the PC has some life left and can possibly be used for low profile computing? The PCs usually sell for around $50-75.

Erik Hundrieser
Los Angeles