Is there a way to tell what my greed machine percentages are for each character without beating Ultra Greed with each?
Unfortunately there is no way to do this within the game itself. It is technically possible to do this via modding the game, but this Reddit thread (from a Binding of Isaac modding subreddit) makes this sound difficult at best.
You could use something like Cheat Engine to teleport each character to Ultra Greed and kill him instantly to check your percentage. If you do this only with characters who have already killed him, you won't undeservingly unlock any achievements. (Characters who haven't killed him have a 0% jam percentage, of course.)