I live in CO and years ago, me and my ex boyfriend filed our taxes jointly because he did not have a job. We never had anything else in our names together and I never changed my name. I am now trying to get married, I don't know if I had a common law marriage with my ex. Will it show up if I did when I go to fill out my application with my fiancé?

Hmm, CO is not the best place to deal with this. See Colorado Common Law Marriage Requirements | Colorado Divorce & Family Law Guide

It does not appear that you met the requirements for a common law marriage, though you did meet at least one of them (filing joint return). It may depend on how many returns you submitted. Before you commit bigamy, I suggest you consult a CO lawyer who can tell you whether you need to formally dissolve that prior relationship.

On a separate note: I don't understand why filed jointly if he had no income to declare. In fact, doing so may have been fraudulent and may get you in trouble with the IRS.