Assuming your playing broadband noise. The largest indicator will be the integration between the sub and your main speakers. You want the lowest delay possible without causing cancelation with the main speakers. This will appear as a dip in the frequency response around the cross-over point with the main speakers. e.g. you have a crossover at 80hz, if the delay isn't in sync with the mains you will see a dip in the frequency response around 80hz. The only caveat to this is if your room has modes at your measurement position that line up with the cross over point, measure the sub alone to find these first.