My pocket radar needs a new battery, so I have not clocked any lately. After you clock 100+ pitches, your naked eye can guess pretty darned good. Most of the pitchers range from 55-70 mph. Outside this range is an outlier. I've noticed the top Major teams have a tough time with 55 mph and less if you change speeds often and location.
Ask their dad and then subtract 10-15 mph from that.
I've clocked a couple dozen AA/AAA (mostly AA) pitchers this year, using a pocker radar. Low guy topped out at 45, fastest strike I've gotten was 62. The latter was from a guy who hit it consistently, though I've also seen one kid who hit 63, but not with strikes. Only 3 who could consistently hit 60+, roughly equal numbers top out <55 as >55.