Naturally, I ask to look at their plastic mounted slides and sure enough, there are the scratches. The best way to see scratches is to hold a 6x or 8x loop up to your slide. Now look through the loop and hold the slide up to a strong light. Now rock the slide back and forth so the light comes through the slide sideways. The picture will go dark but any scratches will show up immediately.
Film processing scratches film in other ways as well.
Dried film seems tough but it is incredibly easy to scratch. Handle it with extreme care. The way I avoid water spots on any film I process myself is to dunk the film in photo flow or stabilizer solution. Then I hang the film up in a Jobo film dryer and allow the forced heated air to push the water off the film. Using regular drinking water contains minerals which causes the water to cling to the film instead of sheeting off.