How do you know how fast your throwing?

Im 15 and am just curious how fast I throw. Ive never been gunned except at some Binghamton Mets games a few years back. Is there any way to find out how fast (or slow) I am throwing?

The only truly practical way is the use of a radar gun
(even then there are variances). But don’t worry about that!
Location, Movement, Changing speeds are all said to be more important than velocity. Most all varsity kids can hit an 85 mph fastball (pretty solid speed btw). Worry about the other parts, velocity will come (gotta keep working though)

Good Luck!