There are line pressure tests that can be performed. The most common cause of this, however, and not just specific to Nissans, is when the reverse clutch snap ring pops out of its groove in the reverse drum. Without the snap ring in place, the reverse drum piston can't compress the steel and friction plates in order to get reverse to operate. The fix for this would be to replace the reverse clutch drum and snap ring, but some might replace the whole reverse clutch drum assembly. This requires removal and teardown of the transmission, which, depending on age and mileage, may make more sense to overhaul or replace the transmission.
Of course, it makes sense to check the basics like fluid level and condition, the shift cable and cable bushings, etc., before condemning the transmission. A valve body issue could also cause this problem.