I live in the rust belt. These trucks rust badly on the cab mounts which makes the cab sink down. Make sure the cab is striaght with the bed.

Check the frame, looking through the front wheel well where the torsion bar attaches, have seen those rust there and also right under the cab. Check rear spring shackles.

Ask the seller not to start it up before getting there, I do this all the time. I like a cold start when I look at a vehicle. You can check for smoke, long crank times, idle issues etc.

Before starting check the engine oil.

Check out the transmission (fluid, shifts, etc), and ask about maintenance history. Look for a cooler.

Listen for the sound of the fuel pump. The noisier it is, the more likely it is to need attention. These GM fuel pumps don't handle low fuel levels well, so hopefully the seller is the type of person that refuels no lower than 1/4 tank.

Look at the sides of the radiator. That plastic gets brittle over time. If it looks new, you're good. If not, look for leaks and expect to replace it in the next couple years or so.

Look for rust on the brake lines. It's probably not a deal killer, but it's something else to budget for or maybe try to use for negotiations.

Some states don't have winter and road salt.

The trucks just go until they explode.

Up here we are just used to a vehicle rusting to pieces before 200k.

2002 Chevy Silverado 1500 Z71  192K still runs good.    Hoping to get another 5 years out of it but rust is starting to be a problem .

Of course it could die on the way home tonight and wouldn't owe me a penny.

If you're in an area where rust isn't an issue, I had a '98 3/4 ton GMC Sierra, had to get rid of it because underneath rotted away.  It had 168,000 miles, top end was rebuilt at 120,000.  Damn thing ran like a top, we miss it!  The new GM trucks are complete SHIT!

GMT400 or GMT800 both have a pretty damn good track record.

My 05 Escalade (which is just a nicely trimmed out Tahoe/Silverado etc) has 200k on it and runs like a champ. No rust either, rare for a Michigan vehicle.

Beware of newer Fords with aluminum heads and high mileage.

A mechanic I know says that when spark plugs eventually have to be replaced, they frequently strip the threads in the head.

He says that most places figure in the cost of removing a head to fix the problem when they quote you a price for replacing the plugs.

And if you do it yourself, you get the pleasure.  Yeah, I know about "Fix-a-thread".

Once you have been down that path, you just replace the plugs every 50,000 miles or so (or get rid of the truck) to avoid the problem in the future.

Rust on the 90s and early 2000s GM truck fuel/brake lines is a major problem.