Rust,

Seat posts, bottom of boot, front sidelights around edge, all arches, all around edges of doors, look between side skirts and body, remove door rubbers and look underneath.

Mechanical,

Blue smoke, rattles, bit slappy is ok, excessive tapping, overheating in general, overheating after a boosted run, gearbox 3rd gear tough to get in, clutch obvious etc.

There's loads mate depending on spec etc