How to check if person is married already in germany

I am indian girl wants to marry indian man in who is living in germany since 10 years. He has just applied for german citizenship.
I am asking him about the documents on what basis he got that normalization but he is avoiding by fighting.
I have been told that few men married german ladies for three yeaars of contract marriage to get it done easily.

How would I know that this person has not married in germany already and if divorced or analysed ?

Whether a married indian man can marry indian girl in india and register it in germany ?
What are the remarraige rules in germany ? I am afriad if he is fraud. Which office in munich can give me all secrete information about married, divorced for citizenship?

In Germany (and most other countries) a "certificate of eligibility to marry" is required before anybody is able to get married. This certificate tells you whether he is currently single, and he (not you, for privacy reasons) can get it at his local Standesamt.
I have no idea how to find out whether he was ever married before, other than by asking him.
If you distrust him, you should not marry him!