Check the paintwerk over for cracks, particularly at the joints. Be sure to tip the bike upside down. Check the rear dereilleur for evidence of a heavy impact with the road.

Look at the forks from side on and make sure they aren't bent back or forwards from what you would expect to be normal.

Rotate the wheels looking for buckles or dips. Listen for grit in the bearings (sound of crunching sand in your teeth) or totally shot bearings (loud noise of any kind).

Make sure all the cables are in reasonable condition. Lift the rear wheel and make sure the rear dereilleur functions across all cogs on the cluster.

Check the brake pads are not too badly worn, and are properly align with the rims. Test the brakes while off the bike. Make sure that the calipers centre properly.

Grab the handlebars and make sure they are fixed firmly. Same with the brake levers.

If you are happy that it's not going to crumble then jump on and ride hard, checking for chain-slip. Most 9 speed gear doesn't last much more than 6000-8000km of hard riding, 9 speed chains 3000km. Give the brakes a good workout too.

Listen for obvious creaks, clicks, rattles, clunks and other unwanted noise.