I don't ask in the moment, but each time they level up I ask my players to tell me max hp, ac/saves, passive perception, spell/class save dc and I write them down for quick reference. If you have their max, you can just track how much damage they have taken using scrap paper and figure it that way.

I dont tell my players npc hp numbers, but I have some descriptors I use. Undamaged, lightly damaged, bloodied (50% or less), ragged (20% or less), dead. These are also a useful scale for opponents who wont fight to the death. For example, the bandits will all fight until bloodied then try to escape or surrender. Hunting predatory animals will try to escape after becoming lightly damaged (predator animals look for easy prey and dont like fair fights), the veteran city guard fight until ragged, etc.