How do you find out if your illegal husband is married or has children in another country?
Ask him. Ask his family, parents, siblings.
Tell him that you would like to visit his country with him as soon as he gets his green card ( usually it is 7 months of marriage. If he refuses to let you come with him to meet his family, you know something is up.
What happens at the airport when an illegal mother returns to her home country with her American born kids leaving behind their illegal father in the US. Will they ever ask where the father was?
You want to marry an illegal immigrant who is not divorced but her husband is in another country How do you marry her?
The illegal immigrant should go back to his or her home country and complete the divorce with his or her spouse. Then you should either get married in that country and apply for a spouse visa, or go visit that country and upon returning apply for a fiance visa and get married in the US after the fiance visa is granted.