One of the easiest ways to do that would be to compare the entered password with the correct password. To do that the entered password has to be revealed and that is what the Firefox password Revealer Show Passwords does. (another way to make sure the right password is entered is to type it the address bar in Firefox and copy paste it into the password field).

Show Passwords adds an icon to the Firefox Status Bar that will change the behavior of Firefox password forms when activated. Password forms are usually showing asterisks instead of the real characters, Show Passwords changes that behavior and turns the asterisks into visible characters again.

All that needs to be done to verify a password is to enter it, click on the button of the Firefox password revealer and compare it with the correct password. All password forms will show the real characters instead of asterisks from that moment on. Another click on the button in the Status Bar will change the behavior again to the default one so that asterisks are shown in password forms.