Income earned during the marriage by either party is community property.
This income retirement and contributions to a 401(k) or IRA or pension.
I am aware that there is a railroad pension that expressly remains the sole property of the party that earned it. However, these sorts of "separate property" issues are rare in the State of Texas. There are a few exceptions to the presumption of community property - talk to a family law attorney to see if your case is one of them.
If some portion was earned before marriage, that would remain your separate property. But the burden lies on the party asserting separate property to prove it. Having a print-out on the date of your marriage showing what was in your account on that day goes a long way in proving separate property.
However, if during the marriage the asset dropped below the starting number, then the lowest number would become your separate property.
You have a bank account on the date you married with $10,000 in it. Two years after you marry the account balance drops to $5,000. On the date you file for divorce there is $50,000 in the account. Only $5,000 is your separate property (lowest balance) and the remaining balance of $45,000 is community property to be divided by the parties or the court.