Deadmen and tell tales are different from each other. Deadmen are wooden, or bamboo posts or poles set up on continuing excavation to have a rough idea of the excavation depth at some point in time. The position is randomly picked or chosen carefully, where the depth is close to average. Now, measuring the depth of the Deadman from the ground level, we can get to know the rough average excavation depth.
Telltales are unexcavated patches left on the excavation. Measuring the height of the telltales from the bottom will give us the average depth of excavation.