Looking at a 2001 slt on sat. what should I look for.

Use the seach funtion and you will find hours of reading.

It might be more helpful if you let us know what you are buying. Automatic, miles, mods?

106,000 miles, 5 speed, quad, 4x4, short bed

If it's a 5 speed, it's the Standard Output, not High Output motor. No matter what he says. There should be a sticker somewhere on the side of the valve cover giving the rating. I'd see how it drove down the highway, the track bar will go and cause drunken wandering, if you can get a mechanic to check the fuel pressure between the lift pump and the injector pump would be good too. At 106k you're just out of warranty on the motor, anything wrong with it when you buy it, and you're on the hook for it.

is there any way from the way the truck is running I can tell if the lift pump is on the way out. And where is it.

turn the key on 3 off 3 times leaving it on and look at the odometer it will flash codes if there is any thing going on