You don't need one of those expensive radar guns. Get a good catcher---you need one---and a good pitching coach with a good stopwatch---and have him time you for several minutes. I remember one time when I was warming up prior to starting a game, and I wound up and happened to throw a pitch that was a good deal faster than my usual stuff. I used it in a game, and the batters couldn't do any more with it than with the rest of my arsenal. I happened to mention this to my pitching coach, and he went and got a catcher's mitt and a stopwatch and had me throw the thing nine or ten times because he wanted to time it. Much to my surprise, he informed me that I had a fastball---a good four-seamer with a lot of movement on it, and at 82 MPH---for a snakejazzer like me, this WAS a fastball.
I wouldn't be surprised if you're pushing, or even breaking, 90 MPH. And the guy you're pitching to doesn't even know how to hold a bat, let alone swing at one of your pitches!