How to tell if head gasket blown?

It really depends on how bad the problem is. If the problem is a small leak in the head gasket or head, then the engine will start and may even run ok for a little bit before it starts overheating. If the leak is bad enough a good deal of antifreeze might be inside the engine and starting it could result in severe damage (this is called hydrolock), and you will likely need another engine if this happens. No matter if you can start it or not, it is strongly recommended that you minimize any driving in it and get it fixed as soon as possible.