Fighting enemies without having any idea about how much or how little health points they have left can be very frustrating, but it's absolutely possible to know this in Kingdom Hearts 3. In order to be able to do this you will need to activate the Scan ability. This allows Sora to scan enemies during combat and tell how much health they have left.
In order to activate Scan, go to the ability menu and chose Scan to be one of the active abilities for Sora. This will consume some AP, but it's worth it in our opinion.
You will not get an exact number, but instead you can see a green bar at the top right of the screen. You can see that the bar will turn from green to dark brown when you deal damage to the enemy, so you get a good idea of how long it will take to beat it.
Some bosses have more than just a single health bar, and this will be indicated by a number of green boxes underneath the green health bar. Once you drain the entire bar, one of these boxes will disappear, and the green bar will be full again. Keep attacking the boss until you have removed all the green boxes and then depleted the entire green bar to beat it.