As said, service records mean virtually nothing. Itemised receipts are better but still not real proof. I always look at the area round the oil and coolant drain plugs - if recently serviced these should be nice and clean. And of course not rounded / damaged as should other hexagons / screw heads. Buying privately you can get a good idea by how well equipped the workshop / garage is.
Mind you the sort of bikes I usually buy would need a service book the size of encyclopaedia britannica......