Lately? I check the mirror.
I used to think I had mad skills, but it turns out that, while I'm a decent storyteller with some creativity, I have poor command of large amounts of rules, and a penchant for letting the campaign "get away from me."
By "get away from me" I don't mean that I try and keep it so closely heeled that there's never any flexibility (see below for railroading grades), but that I'm not very well organized when it comes to keeping track of all that's going on. Couple that with poor ability to manage rules heavy systems (like d20), and it can descend into chaos, or TPKs, or chaotic TPKs in Yuan-ti fortress dungeons.
I used to railroad, but I'm better about that now, and have made a conscious effort to shift from that and keep a more open experience that tells a story with the combined efforts of players, circumstance, and the environment/NPCs I generate. So, still working on that, but getting better. First semester (about, oh, 15 years or so) grade D+, second semester (last two years) grade C+. Teacher comments: Warrior Poet shows promise as a DM, though he needs to remember to let the story unfold as it will, and not worry about it "going" anywhere pre-determined.
I've learned that I work better (as a DM, I'm ok as a player) with rules light systems. I'm just not a good d20 DM, because in my experience it requires significant book-keeping that comes at a real price for me: lack of fun. I'm not very good at math (though playing D&D helps), so it takes me a long time to track all the various things going on "at once," and it's easy for me to get bogged down, which slows up play for everyone, and then my fear that the players are growing bored kicks in, which doesn't help my organization (or lack thereof).

For instance, for the sake of a cool "cinematic" moment in an old campaign, I ended up letting a character take an extra action at the end of a turn, and he felled a bad guy (it was a really cool moment, though! Honest!). Another player later pointed out to me that I had done that (I didn't realize I had, as I had done a poor job of tracking actions/move-equivalents in that round).
So, for now, I think when I DM, I need to stick with rules light systems that allow me to shift on the fly better than I find I can do in d20 (not saying it's not possible to shift on the fly in d20, as I'm sure quite a few people are good at it; it's just very hard for me). Last semester's grade: C-. This semester's grade: B-. Teacher comments: Warrior Poet bit off more than he could chew. He needs to start small, and build up to the more challenging systems. This also applies to his organizational skills, as well. Rules light systems may allow him to build the organizational ability he would need to feel more comfortable in a more rules intensive system. You have to crawl before you can walk, and he may find a better system for his abilities.
Keep in mind with all this I've been playing/DMing for more than 20 years! Yikes! You never stop learning (and you never stop having to go back to the basics every so often!)
Warrior Poet