Push SHIFT during the GRUB hidden timeout to access the boot menu if you already have Memtest86 as a boot option.
You can run the Windows Memory Diagnostic by pushing Win Key + R, and using "%windir%\system32\MdSched.exe"
Keep in mind these tests may take anywhere from thirty minutes to six hours, and it is recommended to let the tests run at least three times over. Let it run overnight or something.