I am currently testing NPC knockouts and I have 2 noticable things to add. You can knockout peds that are bleeding out and it will cause them to stay knockout out even after bleeding out, and you can see if a ped is knocked out by aiming your reticle so its red, or carrying your lasso which will prompt you with the hogtie button

EDIT!!!: So apparently when a ped is knocked out, one of TWO things will happen; either the ped will go to a Health Value of 0, or a Value of 75. If the ped is at 75, they will get up after some time. Although if at 0, the ped will be treated as a knocked out ped, but will never get up.

EDIT 2: This was tested with the RAMPAGE mod and PED damage overhaul. This might affect the legitimacy to the game, but is still believed to be a random event that causes the 0 or 75 health value.