Is there a quick way of telling what level a mob is before you engage it in combat?

No.

Not that I've noticed but the save system makes it moot for the most part, unless you're one of those hard core RP people that quits your game if you die b/c you're playing a "dead is dead" game, heh. If I attack something, I just gauge my chances by how hard it hits me the first time; that generally lets me know of my chances.

Tag it. Now if it hits you, and your not dead in one hit, look at your health. If your over half dead in one hit... that would be an indication that the mob is atleast badass level. So you should turn around a run for the hills!

But seriously, get the guide if you can afford it. It's 650pages... there's a TON to this game. (the guide tells you the mobs level and hp and stuff too)

Go hard or go home. Hit it and see what happens.

also... lol at "badass" level.