[mikejmoffitt]

The difference in signaling is that in an interlaced mode, the final scanline is half-length - Vsync begins halfway through the line, causing the next frame to be offset by half a line vertically.

The monitor doesn't "know", in that case. The interlacing is simply a physical phenomenon. Later digital capture devices must recognize this situation and handle it appropriately. Many assume it's always happening, as progressive-scan 252-field NTSC is out of spec anyway.