I have no idea what you know about cars, but in my limited experience i would say;
AA/RAC inspections worth it if you dont know your way round a car to check it out mechanically.
The list is endless of what to look for when buying a car; however things like paperwork/service documents to show the cars been looked after and also a way for you to verify the mileage (within reason). Find out when next service is due – the person may be selling just before a major (expensive) service is needed. Same goes for cambelt replacement, if necessary.
Hpi check worth it was well – to see if the vehicle has been written off at any point, any outstanding finance on it and i think they can also provide mileage checks.
Gut feeling plays a part as well – if something doesnt feel right – the car, the person selling doesnt seem genuine – then walk away.
HTH.