This question/worry fascinates me.
The classic answer is: First I got form rejections for my subs, then a few comments, then really complimentary personal rejections and then I started to sell.
Hmmm. A nice linear progression that. But I've always sold (lucky old me) and got a few complimentary rejection letters.
I suppose if you're improving you'll make more sales, but as time's gone on I've started subbing more and more to professional press. So that doesn't help much. If I chart the number of sales they're much of a muchness over the last few years. (2010= 7 pro sales, 2011= 4 pro sales 2012 = 7 pro sales) And I'm happy with those numbers. I know that I couldn't have worked much harder.
Maybe I've plateaued? How can I tell? Maybe *le gasp* I'm getting worse.
So how do you tell if you're getting better?