1999 Mazda 626 Engine not working

My daughter was driving my son's 626 and it kept overheating. She kept putting water in the engine but it finally stopped wprking. I put water in it again and it did not leak. The oil looks fine. I took it to a local dealer and they said the engine was blown. I can understand that, no water - no work. My question is that they just looked at it and turned the engine over and the starter only worked. They did not take anything apart or look at any of the parts of the engine. My question is about the degree of serious damage. How do I know the block is seized or it might be some other problem? Are there other heat related problems that could cause the engine not to turn over? I guess I need to know what steps I can go through to finally diagnose the problesm? Thank you for having a site like this to help us.