How can I know if I'm any good?

So, my students tell me that they like my classes.  Some even tell me I'm the best math teacher ever.  But what do they know?  All they know is that I'm the only high school math teacher they have ever had.  That the teacher before me was primarily a biology teacher and didn't really know math so didn't teach it at all, so I must be better than that right?  And finally, that they like me personally (which is nice, but doesn't make me a good teacher.)  They don't know about the material that we don't get to, or the games that are out there that other teachers do but that I just haven't found yet, or how tired I am all of the time and how I probably could have explained a lot of what we just covered much more clearly if I didn't have a million preps.  Their comments are all I have to go off of and I keep thinking to myself- they have NOTHING to compare me to.  So how can I know if I really am doing the best job anyone could do in this situation, or if I'm actually terrible at teaching but personable (I think.  I must at least have this quality if my students like my classes.)  I did though, get an AMAZING e-mail today that makes me feel like at least, maybe I can do this job with some degree of competency and makes me feel that all the time I pour in is worth it.  This is from a former student from last year who is now going to a large university (probably going to major in theology but I wish he would major in math or science because he was so good at it.)

Hi!
I just wanted to tell you that I really miss your math class. I am taking
math 110 and the teacher is nowhere near as good as you are, even with a
smaller class.

Thank you for being the best math teacher I have ever had.

I love my students so much.  They are such good kids and they know when I need an e-mail like this.