The closest I come is to dump the loose ash from the top of the pipe, tamp gently, and see if there's enough left at the bottom to light and smoke. Sometimes I get another round from the bottom of the pipe, and if so, there is not enough dottle left to pick through. When there is a little pile of unburned tobacco at the bottom, I just write it off. If you smoke at all, pipes are the thriftiest way, so getting too cost saving may overdo a good intention.