Once a flight is sold out, it appears that one can look at the seat map only by logging in and clicking in the seat number, which will usually bring up something like a statement that you cannot change the seat and do you want to continue to just lok at it.

Remember to look at later pages or click "show all" to see flights, which are not necessarily listed inane logicl order. Non stops that are expensive can appear in the middle of a long list of crazy connections.