A plumber came out to my house yesterday and had this tool on his arsenal which was basically a 3 or 4 ft long thin (1/2") fiberglass pole with a metal tip on the bottom and a handle on top which makes it look like the letter T. Here's a quick sketch of it I made in paint:
He used it to help find the cleanout for the waste line before digging by pushing this tool into the ground.
I think it would be a handy tool to have, but cannot put a name to it that works in search queries. What is this tool called?