Assuming you already know the basic normal new/used auto buying tips so here's what I look for on used GM Trucks.
Check the tranny dipstick and pay particular attention to the color and the odor of the fluid.
Take it for a city & highway drive and be sure to listen & feel the shifts in the tranny. Leaks, hard shifts or slipping normally are good indications of an trans issue.
Look the body over really good for rust, the rocker panels, cab corners and rear fenders are the common spots for rusting through on our trucks.
Check your AC/Heat Controls being sure it blows correctly on all settings through the selected defrost, dash vents & floor vents when selected. The fan blower, resistor and actuators are know to crap out. 2000 models have 3 actuators, 2 under the dash that are fairly easy to replace and 1 in the top of the dash that requires dash removal to replace.
Good Luck !!!!