It's pretty hard to say for sure.
If your computer will pass a few hours of Prime95 and will pass a night of memtest, but crashes in games with a black screen, chances are strong that a VRM is overheating on the video card.
You might see something with GPU-Z or HWmonitor.CPU-Z
Vital for overclocking, but also for detecting RAM settings.Real Temp
Just measures CPU core temps. Does it well.HWMonitor
Tracks many temps and a few other useful things.GPU-Z
Tracks many GPU tempsmemtest86+Prime95