The last two used systems I bought I was lied to and found that both had
hardware problems that were not noticeable on running the machine and
only discovered later.

What can I do to comprehensively test a used machine I am looking at to
make sure it does not have some minor hardware problem that is not
immediately apparent? Already know about scandisk.

Is there a good free software utility that will run a comprehensive
check? Again I've used a couple of these and still found hardware
problems that did not show on these checks, but they were older
programs. Any helpful advice is most welcome.