I think I've tried a lot of tracking methods in the short few years I've been working on catechism stuff with my kids. It has been a pretty big stumbling block for the perfectionist in me when I get stuck on how to track them instead of just working on the questions.
I tried marking them in the little book I mentioned in a previous post with the date they knew it and their first initial. This was simple, but I just didn't love it because 1. they couldn't see the progress and 2. sometimes I lost the book (mommy brain!). I should have probably also gotten a second book for my second kid, but alas, poor not-first-child-problems. Now, I just keep this book around or even in my diaper bag.
I do know a lot of people use this method and it works for them. Also, the publishers of the book recommend you let the kids color in the numbers (which are outlines) when they complete them, so that's another option.
At one point, I wrote a bunch of the questions and answers on index cards and with sticky tack had one closet door in the bedroom be "Questions I Know" and one be "Questions I'm working on." I did like how visual this was; it made it very easy to incorporate it throughout the day since they were always in front of us. It also made it super simple for both my husband and me to know what questions we were doing. I may go back to this, but it didn’t look as cute as I wanted it to and as I started doing it with two kids, it would have been too much to have all the questions on cards for each kid. Now that I have three kids, I can only image the insanity! But perhaps, cuter hand-written or printed out versions would be nice when we're working on questions as a group. Look for future posts about that!