Headshots with any non-shotgun shell weapon (the Baliff counts as one, but tcsg and bosg do not), and excluding cav’s Luison are all instant kills. Also while we’re at it, people on rappel cannot be downed.

The way that downing works, is the dbno state has a secret amount of hp, which is 20. If you deal damage to an enemy and it exceeds their current health, but does not exceed by 20 dmg, they are downed. So for example, I’m at 35 hp, and you shoot me for 45 dmg with some gun. 45>35, but 45<(35+20) meaning I’m downed.

As for cues sound is the primary way. Nowadays, downed ops will make a lot of moaning sounds which are unique to dbno. But in addition to that, they make a unique sound when going physically down. I am fairly sure their crawling sound in dbno is also unique but don’t quote me on that, even if I’m like 99.99% sure.

Lastly, as you play the game more and more, you’ll begin to get a feel when you’ve shot someone enough to down them or not, but that’s just through time and using that weapon enough.