Once you set your work home position, an interference cube is automatically created, I believe it's a 50mm cube around the taught position. For NX100, this will be cube #32.
Now for the fun part. If you have the Concurrent I/O manual handy, that's where this info came from. There's also a sticky in this forum that will give you a crash course. Anyway...for each cube, the robot defines a specified output that is ON when the robot is inside the cube. For NX100, cube 32's output address is 50117. Now that address can't be used directly in the robot job, but it maps over to SOUT#(88), which can.
So, to find out in your job whether the robot is at work home, you would do the following:
DIN B000 SOUT#(88)
B000 will equal 1 when the robot is inside the cube.
There's one big gotcha to this method however: if the robot is passing through the point, rather than actually stopping there, then I have seen B000 never change to 1 as I passed the point. Two solutions here, 1 is make your cube bigger (in the ROBOT/INTERFERENCE menu) or force the robot to stop at the point, by adding a PL=0 tag to the move instruction.
Scott