If you can get someone who knows about bikes it would be very useful. If you can’t you can do the following. Check the wheels are true by spinning them and check the spokes are tight. Hold the front brake on and move the bars back and forth, any play here or clonking indicates worn head bearings. Hold the crank and check for movement, any play indicates a worn bottom bracket. Eyeball the bike from the back and make sure the wheels line up. Check the tyres for wear and perishing. Finally ride it and make sure the gears shift cleanly and brakes operate well. Are you comfortable on it, not just the saddle but is the position good once you’ve set the saddle height? It’s quite a racy bike so might have a head down attitude. The Boardman is going to be less extreme and more relaxed.
Any wear or out of true wheels aren’t deal breakers but can be used to haggle down the price.
Good luck.