Free to Download and use Video Card tester (GPU)
Hello, first thing to do is to install latest updated graphics driver for your video card from here, then download video card testing software from here and start it.
Select : Resolution to 1280x720, 8X MSAA and start test with Burn-in
Now, wait to see if artefacts will appear on your screen and if yes, stop tester from top right corner and lower your GPU Clock
If nothing strange apear on that donuts shape after 5 minutes, then everything is ok and you can adjust more overclock or leave it and use it at current overclock.
If you want to be 100% sure that video card is stable, set resolution to your native resolution (see here guide) and leave it for about 15 minutes.
If after 15 minutes is clean, then video card is very stable
Clean and stable is like this :
Error and overheating video card is like this :
So be careful, and if you have artefacts, lower GPU Mhz to default.