For testing the Black HAT soldering my method is to use a LED bus I have lying around to test 4-6 pins in one go, which is essentially a continuity test but as Jon pointed out it's pretty crude method.
... the Black HAT was the very first thing I ever soldered and really I only consider myself good enough 6 months down the line to tackle another one, 120 pins to get right (a significant number being ground) is no small feat.
Honestly, the best way IMO for your issue is to eliminate the Black HAT out of the equation until you figured out whether the fault is not elsewhere. If you come to doubt the soldering look up the pinout for your hardware and touch up the points when problem arise.