How can you test how quickly you hit the ball?

Question

some female tennis players are known to have like 80/70mph second serves, so how can i see if i hit the ball quicker?

Answer 1

The only way is to use a radar gun, you may buy one or in major tournament stadiums such as legg mason tennis classic they have the equipment and do so for free.  

Answer 2

i went to U.S. Open many times, one of their festivity stands, i recall, was this serving cage equipped with a radar gun, anybody can walk up and take a swing. the radar will show you what is your serve speed.