We only have a bar showing the relative HP left. Why not let us have the number shown, same as that on the resources.
Use a save game editor to view the exact amount of hp your dwellers have.
to be honest all you need to know is that to obtain the max amount of endurance you need to train your dwellers to max E at level 1. You gain health per level extra based on your Endurance (not counting gear, though I am no longer certain of that).
I found that there is a sweet spot when sending out dwellers exploring. When they reach level 30-32 ish, they can take a reasonable amount of punishment before using a stimpack. If you have a max E trained Dweller reach that level, giving them 25 stimpacks is enough to keep them outside until they reach level 45 easily. If you give them a good weapon, level 50 is possible. Dwellers gain full health when leveling up, so thats useful to remember.
Have 1 dweller that you are ABSOLUTELY sure of having max health (10 endurance, trained from level 1 onwards). Place that dweller in the first room after your entrance. Now fill up the rest of the room with other dwellers, take off any E gear they have and weapons. You can either rush the room or wait for raiders/deathclaws to attack. After that it's a matter of paying attention to their health bars; any healthbar that drops faster than the first dweller's healthbar is not maximised and can be removed.
After that, put a Z in front of the names of dwellers that have passed your test (so they dont appear at the start of the list of dwellers when you sort by name). Kick out dwellers you arent satisfied with. Sure it takes a while, but if you want to optimise it that badly, its a pretty solid strategy.
Alternatively you can rush whatever room they're in until they are hit by either molerats or radroaches, then see how they last.
Endurance from gear does count, so that heavy wasteland gear with +7 endurance is worth it's weight in gold. Some guy over on Reddit did a https://www.reddit.com/r/foshelter/comments/3ibkjj/endurance_affects_hp_only_during_leveling_not/ of how earning hitpoints works.