Common law marriage is like a tattoo: easy to get, impossible to get off. Okay, maybe not impossible but you get the point. There are only three requirements to having a common law marriage in Texas. You must live together, you must have an agreement to be married and you must "hold out" as husband and wife. The "holding out" requirement is a legal term encompassing many factors that generally show the court you behave like a married couple. To get a tattoo, I think you have to be of age and probably under the influence.
As easy as it is to meet these general requirements, especially relative to the cost and time involved in a formal wedding, proving them can cause all kinds of problems in a divorce. And for those that are lucky enough to pass away married, there can also be problems proving the marriage existed in probate court.
The problems are in proving the three subjective and fact intensive requirements. There is no exonerating DNA-like test in this regard. Living together is relatively easy to prove under normal circumstances, but what about the requirement to have an agreement to be married? To prove this, a party must show that they intended to have a present, immediate, and permanent marital relationship. People are living together more and more and behaving like married people but that doesn't necessarily mean they have agreed to be husband and wife permanently. As Dr. Laura would say, you need a ring and a date. On the other hand, a ring and a date evidencing an engagement could defeat the agreement requirement as the definition of engagement admits the parties are not yet married.
Convincing the court that the parties "held out" as husband and wife could also be difficult. For example, if the parties don't refer to each other as husband and wife in public, don't file joint tax returns, or don't list each other as spouses in medical or legal documents, this requirement could be defeated. But if they owned real property and debt jointly as husband and wife, then they might have a chance of proving the "holding out" requirement.
The hard to get off part is the fact that once common law marriage is proved, it is just as difficult to divorce as it is when you have a formal wedding, if not more difficult. If the court finds your relationship met the three requirements, your assets and debts will be divided in a just and right manner. Most people consider this scenario to generally mean your assets and debts could be split 50/50 but this is not necessarily the case. See my blog "5 Factors Affecting the Just and Right Distribution". For these reasons, common law marriage should not be entered into lightly.