There is also a baggage check serving multiple airlines on the B side at the front of the rental car return area on the first floor of the parking garage. After you drop off your car, walk toward the terminal. The bag check desk is located a few steps to the right of the Alamo check-in booth in the garage. Check your bags, cross the road and you are in the terminal. If you want to use it, you should check your bags at least 90 minutes before the flight.
There may also be one on the A side, but I have not been there.