Is to bring it to the grocery store. Go get two five pound jugs of water then walk over to the produce section. The jugs of water should weigh an exact weight (check online to see how much five gallons of water weigh). However, I place them on the produce scale just to make sure. Once I know the exact weight, I then place it in one of those flimsy plastic bags they give you (no added weight) and weigh it. I weigh several 5 gallon jugs to to account for potential variation. Last time I did this my scale was dead on. I need to do it again soon though just to make sure its still on.

And to play devils advocate, angles, sunlight, type of camera, zoom distance, and size of person are all very influential on how big a fish actually looks in a photo. So many times just cause a fish doesnt appear to weigh X amount, in reality it actually did. But I still agree, lots of people are way off the mark.