I always ask them to check that it is not the insides, when they invariably say they have and it is on the outsides, I tell them I am happy to come and put right, but, if I come and find that it is indeed on the inside there will be a £10 call out charge plus my time on top.
From that they mostly check properly and confirm it is on the inside, a couple have insisted it is on the outside and then ended up paying the charge.