If you feel you need to pre-board, ask the agent at the ticket counter at the gate. Most airlines now don't announce pre-boarding, but will discreetly let those pre-board that need it. If you're installing a carseat, you may have a good case to pre-board, but its really up to the agent.
On Southwest, pre-boarding is allowing for families travelling with children age 5 or younger, although it can be suspended or restricted at times if there are a lot of families on a particular flight (particulary out of Orlando).