Stellar bars in galaxies induce morphological features, such as rings, through their resonances. Previous studies suggested that the presence of 'dark-gaps' around the stellar bars, regions of the disk where the difference between the surface brightness along the bar major axis and along the bar minor axis are maximal, can be attributed to the location of bar corotation. Here, we test this claim using GALAKOS, a high-resolution N-body simulation of a barred galaxy. Contrary to previous work, our results indicate that 'dark gaps' are a clear sign of the location of the 4:1 ultra-harmonic resonance instead of bar corotation. We show that measurements of the bar corotation can indirectly be inferred using kinematic information, e.g., by measuring the shape of the rotation curve. We demonstrate our concept on a sample of 578 face-on barred galaxies in MANGA with both imaging and integral field observations and find the galaxies sample likely consists primarily of fast bars.