Arm velocity’

Find a guy with a radar run and throw a ball.

I'll try for a less flippant response. You're not going to be able to know your max arm velo without a radar gun. A Stalker Sport 2 isn't crazy expensive and when you're done with it you can probably sell it and recoup at least half your cost. My son is your age and we sucked it up and bought the Sport 2, though he's transitioning from catcher to RHP.

If all you're looking to do is figure out relative arm speed improvement, or what is impacting your POP time, i.e. release time vs velo, then my son and I had some success using Hudl Technique on an iPhone 6s (we still use it to break down his pitching mechanics). It's a free download and with Technique you could capture a video of yourself throwing down to 2nd. It'll take a few attempts to figure out where to stand and how to manipulate the phone (and the zoom) so that you capture both the POP and precisely when the ball hits the target.

Once you capture the vid, you can scroll through it frame-by-frame against the built-in timer and figure out from body movement initiation how long it took to release the ball, e.g. 0.65 seconds. You can also measure how far release point was in front of the plate and use the traveled distance to calculate average speed. Let's say your POP was 1.90 sec. Your release was 0.70 sec so travel time was 1.20 sec. Let's also say you released the ball 2' in front of the plate, which means that it traveled 127.3' minus 2' = 125.3'. 125.3'/1.20 sec =  104.42 ft/sec x 3,600 sec/hour x 1 mile/5,280 feet = 71.2 mph average speed.

Obviously doesn't give you max speed at release, which would probably be closer to ~82 mph in the above example (drop off due to air drag). But it would allow you to gage overall speed improvements and breakdown whether further POP time improvements are connected to arm speed or release time.

I recommend an iPhone 6s or better. We tried it on my Samsung Galaxy S6 but slo-mo video capture is only 120 fps, and you really need the 240 fps from the later iPhones to eliminate blurriness. It's difficult to capture the arm moving at only 120 fps as rotational velo from arm layback to release is very high.

Using Technique, or other similar apps, is a great way to get accurate POP times as it eliminates fast thumbs on a stop watch as the camera and built-in timer doesn't lie. It's also a great way to improve someone's consistency using a stop watch. My own stop watch accuracy improved considerably - from what I wanted my son's POP time to be vs what his POP time actually was - once I started benchmarking myself against the Technique vids. It's also a great way to capture accurate game POPs, which at the end of the day is what matters. Good luck.