As Brad said, the reason Cat gets a bad rap around here is alot of us are shade-tree mechanics and Cat parts are spendy.
Things I look at:
Case Of Beer... Or C.O.B... Coolant, Oil, Belts. No coolant leaks, no oil leaks, oil level, belts but loose, cracked or frayed.
Pull the oil dipstick... Is it honey or black tar or coffee? Take a sniff, does it smell of diesel fuel in the oil pan?
Underside of engine and transmission... Is it caked with dry oil mixed with road grime? A little coating of oil is normal but if you can break off chunks with a putty knife that's no good.
If it has a clutch then it's not an automatic... So basically just can you drive it and is it geared high enough for your expectations.
If air brakes, start it up, let it build pressure, listen for leaks. There's a whole air brake test procedure but basically as long as you can't hear air escaping. Brake pad linings need to have a measurable amount of wear remaining.
Tires... Basically tread depth, uneven wear, cuts, bulges, visible cords, separation of tread from sidewall.
Hope this helps.