How do I check that ?
If you have a multimeter, check for continuity between xlr pin 1 and the ground sleeve of an rca connector. Otherwise check with the manufacturers.
However, the most common inconsistency in xlr pinouts is that the signals on pins 2 and 3 are interchanged, which would not affect hiss levels. It would just invert polarity, sometimes referred to as "absolute phase," which would have minor or negligible effects on many or most recordings.
Also, check that continuity does NOT exist between pin 2 and ground, and between pin 3 and ground. A near zero resistance between one of those pins and ground would indicate that the unit does not drive the xlr connector with a differential pair of signals. I would be very surprised if that were the case, but if it were it would go a long way toward explaining the hiss.
Regards,
-- Al