When playing 1v1 on maps with four starting locations how do I know if any of the starting locations are disabled for 1v1?
It depends on specific map implementation. For example some tournaments use Metalopolis map where you can spawn only on cross locations. If you wondering about ladder maps - spawn conditions usually described in map notes. You can also check maps description on teamliquid wiki where spawn location rules of different implementations are described (i.e. metalopolis)
P.S. there is one trick about finding opponent I discovered a while ago youtube video