This is actually a good question.
The nether is divided into quadrants of 430x430 blocks. Within these quadrants only 1 structure (bastion or fortress) can spawn. So if you see a fortress at x= 250 z= -300, (which is positive-negative), then you know there can't be a bastion in the positive-negative coordinates, unless you travel past 430, -430. Here's a picture I drew. Hope this helps:
O-_o