A little bit more scientific, but still redneck a bit is to use a bathroom scale. If your trailer is light enough that the 15% tongue weight is in range for the scale you can do it direct. If not, put the center of the scale two feet to the left of the tongue with a small piece of 2x4 or similar in the center. Then put a chunk of wood 2 feet to the right of the tongue roughly the same height as the scale and wood. Then put a 4 foot board between the two and set the tongue on it. Whatever the scale reads, double it and that is your tongue weight.