the size should be on the tyre like on the wall it should be like raised nad moulded. it should say the make aswell. check that they have tread on them, inthe uk tyres have to have 1mm minimum. ask him what bike they came from. look for signs or wear and balding. you might get the cash you might not, but they might be hasstle to sell. why not give him a few weeks to sell em and get the money then with a bit of interest