How do I know if we are legally married if his divorce decree was not signed?

If you obtained a marriage license and participated in a ceremony before his divorce was final, you are not legally married to him because he did not have the legal capacity to enter into the marriage with you. All you need to do is marry him again. As far as the house is concerned, you need not be married to someone in order to jointly own real property with them. The fact that you may not have been married at the time you purchased the home should have had no effect upon the passing of title to you.