How do I know how strong the enemy is?

[eXeel]

If you right click the mob group, you can see "health", I believe it is called. That is the damage each mob in the group can take before it dies.

I don't understand your question. Don't you see Attack, Defense, Health, Damage etc, when you right click a mob?
And when you hover over a mob, ready to attack it, doesn't it show how many will die (5-7 for 800-877 damage)?

[Dimension]

Press Alt and you can see the health of your enemy. This is helpful for enemies with much HP like dragons, giants etc.
You only need to press it once, and it should then show up for the rest of the game.
If this is what you were asking about anyway.