Ensure that your bag is what you can handle. If you find yourself hunching over or struggling to stand with it on your back comfortably, then these are signs that the bag is too heavy.
In reality, you don't know whether you have "overpacked" until you're on the trail then it's too late. You could make that discovery 100 feet in, or 2 miles in.That's why we advise following the 10C's and 5C's of Survival and carrying multi-purpose gear, so you only carry what you really need.