A good indicator - does the heater work?
A guage isn't usually prone to failure.
If the heater isn't nice and hot after say, 5-10 minutes of driving, and is instead cold or lukewarm at best, I'd say the temperature guage is probably accurate, and you've got a dodgy thermostat. Happened in my old car. Fortunately, replacing a thermostat is usually quite easy, and a new one to buy is probably only around 20, 30 dollars. If it's never been done, and it's an older car, it might be worth just changing it anyways.
Another indicator - is the guage more likely to rise up on a hot day, rather than a cold one, or when the car is sitting idle for a long period of time without much airflow to cool it down? (Parked, or in Traffic). Basically, the guage might work, but its reading will flactuate a lot, rather than more-or-less sticking to one place.
Otherwise, if the Thermostat is known/presumed good, and the guage is just constantly dead, it may be the sensor, the guage itself, or the wiring inbetween. Sensors can usually be tested by removing them, and putting them (in this case) a container of hot or boiling water. You could possibly have it still connected to the cars wiring, to see if the guage registers. The 'proper' procedure might be to dunk it in oht water as before, and use a Multimeter to test it. Depends on the sensor.
Being a Nissan, it's most likely Resistance based, so you'd put the MM to Ohms. In cold water, or just sitting dry, it will be cold, and should register high resistance. The hotter the water, the less resistance there should be, which the MM will show.