Simply put, I don't see any way to gauge how much health a follower has so I know when to heal them. In Oblivion, a health bar showed up when you placed the crosshair on either an enemy or an ally, why would they leave this out?
Given the usual caveats regarding using computing systems and variances of results...
In my experience so far, from release through the 184.108.40.206 update, I have not noticed any consistent pattern at all (sometimes it appears but normally does not). This includes:
- the bar appearing when in combat or not
- when their health is not 100% or not
- when they were recently damaged or not
- whether the player's char is in combat or not (I thought for a while I saw a pattern here... sadly it was not repeatable)
- when I am healing them, buffing, etc... or not, lol
Note: My rig is fairly high end and the settings are almost all set at the highest levels. As mentioned above, this has not changed at anytime though the different versions from final release which also includes before and after the "evisceration of larger address space, including use of more than 2 gigs of memory" as well as using the 4gb launcher.
If you're playing on the PC, the Floating Healthbars mod can add floating healthbars to any actor engaged in combat.