how to check if the headgasket is in good shape before buying?

The best way is to go to NAPA and get a combustion gas test kit for the coolant (i think they may have it listed as BLOCK TESTER) its about $50 and you can test multiple cars with it, make sure you start and drive the car for a little while before preforming the test so you give the coolant a chance to circulate and collect the exhaust gasses.

and :welcome: to the wonderful love hate relationship of northstar ownership

If the car you're looking at has a Northstar engine, take it out and drive it aggressively at city and highway speeds, performing a couple of strong, dedicated rolling accelerations from 5 mph to 75+ mph. Keep an eye on the temperature/gauge. If it goes above 12 o'clock or 208 and stays there for any appreciable time, walk. (If the car is electric fan cooled, the temp WILL go to about 225 in stop&go traffic, the fan(s) will come on, and it should cool back to ~210. If you have some A/C function turned ON, fans will run all the time, and you should then see NO gauge movement.)

If, under the above conditions, the car boils, loses coolant, steams, hiccups, poor idle, exhaust smoke............walk.

thanks a lot guys, are you saying as long as I don't hit stop and go traffic, the temp should never go past 12:00 ? how do i know if a car is electric fan cooled, (and, just for my knowledge, what are they if they aren't electric fan cooled?)

They all have 2 electric fans mounted right behind the radiator. Temp should stay at 12:00 even in traffic.