At this time there is little guidance for what PCs know and do not know about an opponent's HP.
Coming from 4e, I generally give my PCs an indication when the creature is at half HP (bloodied in 4e parlance). But this is not required and is not mechanically explicit.
It's worth noting that we can infer that characters are at least somewhat aware of something that informs meta constructs like HP, since the Battlemaster Fighter has an ability that allows them to see an opposing creature's current HP (among other meta elements).
Overall I find it inadvisable to make explicit statements about HP totals for enemies and PCs alike in the game. This is a level of granularity that outside of very special abilities, PCs and NPCs alike are not likely to know at any kind of specific level. That said, general awareness of HP totals (At the very least as a quarter, half, three quarters or full construct), is probably something creatures can and should be aware of on careful observation (perhaps a knowledge check of the variety appropriate for the monster/PC).
However, I would also rule that a character is fully aware of their own and their allies exact HP totals. Again the awareness of the meta constructs in play, and also the fact that generally once past the very lowest of levels, PCs are quite experienced combatants, indicates to me that it's very likely they are quite in tune with their bodies. But this is definitely a table level decision.
It is likely that the DMG will carry advice on when and how to give this information.