Engine
Check for oil leaks, the cam cover is a common fault, but only about £3 to fix, so don't worry about that.
The GT engine taps quite a bit on idle, so also don't worry about that too much.
Boosting, drive along in 2nd gear at about 1500rpm and floor it. It'll take off slowly, but when the turbo spools up at about 2750rpm it should pull well, and boost into the red for a split second, then drop to about 3/4 on the gauge.
Check for mayo under the oil cap.
Check for smoke outta the exhaust on idle and boosting. A little condensation is normal on the GT's in the colder weather.
Body
Check for any dings and scratches. Also check for any overspray, and under the carpet in the boot.
Check the rear arms by looking at the back of the car. The wheels should be parallel (|---| not /---\)
I'm sure other people will fill the rest in.