What are the signs of plantar warts?

Several. Pain on direct pressure and side to side pressure of the growth.Seeing small black heads in the center ofthe growth.
Pain, callus. Plantar warts are usually on the bottom (sole) of the foot, and are usually surrounded by callus tissue. Often they are painful to squeeze, and they disrupt the natural skin lines.
Skin line disruption. Along with the other answers, there is noticible disruption of the normal skin lines of the feet when encountering plantar warts. Callouses will have skin lines running through them, but warts will have a "stop-and-start" ditsribution to the skin lines.