Equip a pristine plate carrier and have a friend shoot you in the arms (avoid torso damage by angling the shot if necessary).
Repeat without plate carrier equipped and observe the difference in damage, shock, and bleed to the player. Also observe that the plate carrier is damaged by shots to the arms (even when the torso is fully avoided - no accidental splash damage).
We tested this with .308 rounds/Tundra and it seems pretty conclusive.

We made a video demonstrating the difference in arm damage with and without a plate carrier equipped, here:

https://www.youtube.com/watch?v=uJUZj9pPrro

The first half of the video shows the plate carrier arm protection effect, it takes around 10 shots of .308 to the arm to down the player. In the second half, the test is repeated without a plate carrier equipped. The player goes down much faster in the second.

The plate carriers we tested had pouches and holster attached, we didn't test without attachments equipped.
We also tested with a stab vest - this seems to work the same. In other words, arm bleed and health damage were reduced until the vest became ruined.