How does one check for cracks?

I am considering reactivating a Fawcett wood burning hot air furnace with electric back up that was in the home when I purchased it. Research I have done to date suggests that this is a reasonably good unit. Is there a way to check for cracks in the burner? Are there other considerations for me to explore in terms of safety and efficiency? I assume that since it does not have a grate, sand should be used as a base. All comments and responses are appreciated.
David

Thanks for that advise. I had a liner put in that chimney when I bought the home as there was creasote leakage throught he tiles. Just have not used the burner since.