You don’t have to restart your computer and go into BIOS to find out whether your Windows 8 or 10 is booting in UEFI mode or Legacy mode. You can do so while you are still working in Windows and here is how.
Press Win + R to open the Run command box, type msinfo32 and hit Enter.
It opens the System Information window. Under System Summary, you will easily identify your Windows boot mode from BIOS Mode line.
Here is if it’s Legacy:
And here is if it’s UEFI:
If you see it’s UEFI, you will know at least one thing. The boot partition must reside on a GPT disk, which means by turning off Secure Boot and switching BIOS Boot to Legacy Only your Windows will not be able to boot. If booting in Legacy Only mode is the only way to get your applications working in Windows 8 or 10, you may end up re-installing everything from scratch, since converting a boot disk from GPT to MBR is difficult without wiping out all old data.