If you mouse over the enemy target, you´ll see the healthbar with numbers.

Like: 3746/4395

Use the tilde key (~) to cycle through various healthbar settings.  If I remember correctly the cycle goes something like:

none

self (and minions)

allies

enemies

demigods

all

The healthbar displays aren't linear reflections of health.  I think they are logarithmic.  So that means when the healthbar over the enemy DGs head is almost empty, he's probably at about half health.

Some people don't like it, I think it works.  It makes it much easier to pick out which enemy DGs are nukeable kills and which you shouldn't bother with.  

Just remember, when the health bar says they're almost dead, that means if you nuke 'em  you'll probably  kill them, never just tap someone on the shoulder and expect them to die. 

Yes, you can do it.   hit tilde ~ and it'll show you the health of the enemy.  Hit tilde ~ during in game a few times to toggle between tower, enemy, allies, and demigod only health bards.

They're not logarithmic, they're just broken. When you zoom in, they accurately reflect percentages. When you zoom out, they're incredibly inaccurate. It's a bug...

Hmm, the health bars always seem to show up for me I think.  What's annoying is that these aren't in the settings or they aren't saved.  So at the start of every game, I have to hit tilde 4 times. 

Or they should just allow UI mods.  They were enabled in beta but not sure why they disabled it.  Custom UI mods allow you to make those bars bigger, display mana, and anything else.