You can VATS them, without taking the shot, to see how much health they have. Give them buffout. All your companions heal to full strength after combat. So if someone is near death, pick a fight with a radroach, and they will be back to full health.
I was always curious as to why Charon would walk around with full health(or pretty close to it) after a major battle against both Super Mutants and Talon Company Mercs! Thanks for the explanation.
I know that the companions will use Stimpacks, as I always keep Charon's at 20 and have had to replenish it from time to time.