The tester will test 6-pin PCI-e power cables, it's quite versatile. However, if the power supply is marginal or not up to powering your system, it may not prove much to plug in a PCI-e cable to a tester, it will probably work fine with little load on the PSU. The issue is more likely that under heavy gaming your system is drawing more than the poor-quality PSU can put out. A tester like I linked to will not be able to test that, you would need scopes or probes on different parts of the PSU during your gaming session, sort of what sites like Tom's does when testing PSUs under load. I don't think that's something you're interested in. Just buy a brand-name, good-quality PSU and stop risking your quality hardware, because worst case your PSU will actually fry your hardware.