There's not a great deal you can do until you ride it and use the gears, brakes etc 'in anger'.
If it were me I would check simple things like: are there any scratches to the paintwork or dents to the frame. Spin the wheels to check they were true and the brakes weren't binding. Check the movement in the brake levers, that this isn't too excessive. You could lift the back wheel and check the gear change, but that won't prove them fully. Check the cranks and pedals for play.
Like I say, its not until you get to ride it properly that you may spot problems so just check the simple things first, particularly the frame. If you can ride it in the car park if they have one or just take it round the block then that may be worth it to save you a trip back over something simple.