Datasheet

There is an output sentence that will tell you the status of the antenna. $PGTOP,11,x where x is the status number. If x

is 3 that means it is using the external antenna. If x is 2 it's using the internal antenna and if x is 1 there was an antenna

short or problem.

On newer shields & modules, you'll need to tell the firmware you want to have this report output, you can do that by

adding a gps.sendCommand(PGCMD_ANTENNA) around the same time you set the update rate/sentence output.