Texas is one of the few states that recognize common law marriages.
Here is a link to Travis County's website relating to common law marriages which may be of help: http://www.co.travis.tx.us/dro/common_law.asp
DISCLAIMER I do not practice law in your state. This answer is provided solely for general informational purposes only. The answer is intended to give you an idea where to go for more information that may be more specific to your own situation in your own location. This answer does not constitute legal advice, create an attorney-client relationship, or constitute attorney advertising.
Filing together would definitely be some evidence of a common law marriage. Especially if the return is going to be a married filing jointly. While the filing just by itself is no 100% proof of a common law marriage, living together, and presenting oneself as husband and wife to others would most likely make this a common law marriage.