Find the recorded snow load for your zip code by consulting the local New Jersey permit issuing authority in your region. This is the maximum snow expected to fall in your area.
Measure the distance from the peak or your roof down to the edge to find the "run" of the roof. Your roof might have a run of 30 feet, for example.
Measure the vertical distance between the peak of the roof and the edge. This is called the "rise" since it's a measure of how far the roof rises.
Your roof may have a rise of 10 feet, for example.
Divide the "rise" by the "run" and convert the fraction to a ratio of 12. In our example 10 feet divided by 30 feet is 1/3, which when multiplied can be converted to 4/12.
The roof pitch would be 4:12.
Use a calculator like the Cornell University calculator. Type in the "ground snow load," terrain, exposure, roof type, pitch, and run.
Assuming expected ground snow in your area of New Jersey is 30 psf, the snow load on the example roof with a 4:12 pitch is 2079 psf.