The bottom line here is... YOU DON'T (verify PP plane speed, that is)!

You're better off from a legal standpoint not having a way to measure speed, just like you're better off not having that "BEWARE OF DOG" sign on your door. By having a radar gun, you're admitting that the plane can go too fast, just like in having the BEWARE OF DOG sign, you're admitting that your dog is likely to attack someone. Either one makes proving negligence a slam-dunk for the plaintiff's lawyer.