West moving Tropical Waves always have their negative vorticity out in front of the axis and positive vorticity behind the axis. In the upper level and dynamic systems moving west to east...generally positive is out ahead of the trough/front axis and negative behind...so looking at it; negative is always on left side of any trough axis, positive on the right regardless of what direction it is moving but that's not always the case especially in higher latitudes where upper lows generally rotate around a broader low. Just remember in the tropics, the action is behind the wave, not in front....