I look to see how other people do things. For example, if I think I am an "imposter designer," I look at reputable designers to see what they are doing, and how much of that I can also do. I also look at designers who seem equal to me, to see if they are good.

I also try to learn the flaws of people I admire because that helps me realize that when I am different than someone, it does not mean I am necessarily worse.

Also, Imposter Syndrome is a symptom in the "fear of rejection" category of pride. Reluctance to ship is in that category too. This is a little like Rejection Therapy, but you can beat both those symptoms and hopefully hack at the root if you resolve to put work out there until somebody with authority makes you stop. Most likely they won't, and you'll get the feedback you need to know that you are adequately capable to do what you do.