All Duramax's are able to start with the turn of a key. No need to wait for the glow plugs to light up like the older dodges, and chevy 6.5's. The way you'll know is when you go to crank it the truck wont start and just crank. Also the glow plug light (looks kinda like a spring) will stay on when you crank it.
However, with you 08 I doubt you'll be having problems for a while
