I think (?) that a picture of the plane when you select seats will identify the bulkhead rows. I also think that if you pre board or if you're under 14 years old, you can't sit in the bulkhead seats. So if you choose a bulkhead seat beforehand, the system should pop up with a screen asking you if you're eligible to pre board and if you're under the age of 14. If you answer "Yes" to either question, the system should refuse to let you pick the bulkhead.
Also, let me ask you a question-did you say that you moved up from one seat to another that was further forward at no additional cost? If so, then the person who told me otherwise was wrong. When I flew Southwest on 2/4, someone told me that Southwest won't let you move your seat forward without paying, although apparently lots of other airlines do. On Southwest, you can move parallel or back further if seats are empty, but not forward, they said.