Two types of concrete posts are encountered in benchmark hunting.
One type is a concrete witness post. Generally these are 3 inches square and around 2 feet tall. Inscribed letters such as USBM are often seen on them.
Another type is a monument for a benchmark disk. They are usually 6 inches square and project anywhere from a few inches to two feet above the surface of the ground. The disk is mounted into the top.
Notes: You will find many concrete posts in cities. Many of them are property post (property line) which often location at corner of blocks. Some will have cap. Also state highway department set concrete monument at edge of road as right of way limit.