How do I know I'm entering my password in the real decryption prompt?
As far as I know, applications (especially non-root ones) can't modify the power menu. If you long-press the power menu, there should only be the "power off" item as you mentioned. If you see the other items on the menu, you're not in the real decryption screen.

Or alternatively you could input a false password and see if it's accepted without verification, which would imply phishing.