Tools I use to test memorization:
...practice the lyrics to different tunes, actual or made up.
...practice on a single note.
...practice breathing in the middle of every line instead of the end.
...if it's simple 2/4 or 4/4 rhythm, I sing it in waltz time.
And definitely I go back to the original a few times before I leave it after playing around with it like that. If I can do even half of those things with a song, I don't lose track of it when performing.

In support of some other comments made, I always run a mental video or mental tableaux of a story. Several per song usually, because I'm easily bored and need to rotate them to keep the performance fresh. And for initial learning I swear by that method of going back to front, so I always know where I'm going.