How do I know I'm entering my password in the real decryption prompt? I have a Nexus 4 phone with full-device encryption. As I took it out of my pocket, it was displaying the "Type password to decrypt storage" screen. Presumably it had crashed in some way and rebooted, but how would I know this is the real decryption prompt and not some login spoofing malware app?

I powered off the phone and restarted, and curiously, it prompted me for my SIM card PIN first, only then for the decryption password. In any case, the power-off sequence clearly involves software (a long press of the power button invokes a touchscreen menu with only the power off option available).

This is Android 4.4.2, unrooted (as far as I know).
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.