Originally Posted by DarthParametric
It's a mix of scaling for basic enemies with a smattering of set levels for some boss encounters from what I understand.
From what I can see most white/normal enemies are at least 2 levels below the main character, yellow/lieutenant are usually the same level as the PC, while orange/boss enemies are 1-2 levels higher. I haven't observed and all enemies and kept track of the info, but that seems to be the overall trend from those I've checked. (Though some enemies seem to have a level cap since the large groups of darkspawn seem to never go above level 13 for common genlocks and level 15 for hurlocks no matter what level the PC is.)
The Survival skill is pretty handy if you're curious about the opponents. If you mouse over the red dots on the radar or area map it will say what level that enemy is, if you have 4 ranks in Survival.