There are several techniques to find out how heavy your backpack is before a trek. The most common method is to stand on a scale with and without the pack on. The differences between the two numbers is the weight of the pack.
There is another way. For years my dad keep looking in stores at fishing scales. The problem was none of them handled more then like 40 pounds. Eventually when in San Diego I found scales that went up to 120+ pounds. I came to realize that these scales were for sea fishing.
As you can see in this picture, the scale has a loop across the top of it. All we do is slide a hiking stick through this ring and then put the stick between two shoulders, two cars, rafters of a building, or what ever was available. Now hang the pack off of the hook and you get the packs weight.
Once you know the packs weight, decided if it is more then a third of your body weight. If it is you should reduce its weight. But that is a discussion for another post....