Ah, good question. When you're in the "question view" for each problem, you can click on "show solution & statistics" and there you can see the percentage of users who answered correctly, plus the percentage of users who chose each answer choice. That's a good gauge for difficulty (which isn't quite the same thing as the "
b-parameter" that the Item Response Theory CAT system uses to determine which problems are the best fit for each user, but it's close enough and easier to understand).
As you analyze your errors, I'd also highly recommend two things:
1) Check out this blog post about assigning priority to things to work on:
https://www.veritasprep.com/blog/2014/06/gmat-tip-of-the-week-woulda-shoulda-coulda-how-to-analyze-your-practice-test-results/2) Make sure that you're asking yourself WHY you missed the problem. Was it because you didn't know the concept? Or was it because of a trap answer that baited you? That's why I love the question statistics as a tutor - if someone picked a very common trap that a huge percentage of other users also picked, it's probably not because they didn't know the formula or rule, but rather because they didn't think as critically as they should have when looking at the trap. So make sure that you're not just labeling a problem "geometry" or "modifiers" but instead thinking about why you made the mistake you did.