can a fiance be held liable if soon to be husband is sued in a civil action

There are simply too many variables to provide a focused and thorough answer to this question. Sued by whom? for what? when will you be married? what state? etc., etc.

At the very least you will want to consult with an expereinced estate planning attorney in your area with regard to ensuring that liabilities of your fiance do not become your liablilities when you are married. Prenuptial agreements, marital property agreements, trusts and other estate planning procedures can be implemented to insulate and isolate your personal assets from potential liabilities of your fiance.