Take someone with you who knows about cars.
Check the length of the MOT, the cost of tax, the depth of the tyre treads.
Check the brake pads and all switches and levers.
Plug something into the ciggie lighter and the USB if it has one.
Check it has its spare tyre or inflation kit and tools. Also check for a locking wheel nut if applicable (alloys).
Check for service history and the book pack. It should have its key and radio codes.
Make sure there are two keys.
Check that the dealer has the V5 registration document. Ask to see it.
Check the paintwork over. If it's dirty say you want to see it clean. Check in the dry, rain makes everything look great.