Err dead easy, you inspect with mate looking for the obvious, saving yourself any costs if a shed, then you negotiate and agree sale with vendor subject to successful inspection (maybe put down a deposit fully refundable IF inspection throws up any material issues not disclosed by buyer or with rectification cost greater than say £250 - put this in writing) better still if you can get a sold subject to inspection without deposit, then you take to local VW dealer, for inspection, you can do mileage check and HPI on line yourself , easy to do, ask dealer if he will do dealer check on mileage etc print out of VAG history (he may do if you ask nicely, may need to get owners permission) if clean do deal, if any issues the renegotiate. Simples
Substitute VAG main dealer with knowledgeable specialist if possible and one nearby with good rep, won't get VAG history check but much better check over IMHO.
I have done this with a number of cars and works well.
If low mileage car then also ask for ECU hours interrogation to check against mileage, SO MANY high value cars being electronically clocked it is unreal, they can't wind back hours recorded on ECU though and if you see a 20k mile 3 yr old car with an average MPH of say 70 walk away VERY FAST. This is becoming common cars such as Porsche, not sure about VW's though, but bear in mind it costs £70 to get mileage reset !!! Caveat Emptor - Buyer Beware