Benchmarks often become covered by either washing of soil or the accumulation of vegetation that is decaying into soil. After you have measured to a likely spot, if you have a metal detector you can search the immediate area for a better idea of where to probe. With or without the metal detector, after you have your best location you will want to poke down in the dirt to see if you hit something.
A long screwdriver, a camp fork with tines removed, an old golf club shaft, or similar probe that is easy to get a grip on will let you see if there is something solid down there.
CAUTION: there are a lot of buried utility cables out there, and in most places you are not supposed to dig in your own back yard, much less along a roadside, without calling the one-call locator service first. That means that your probe should not be sharp enough to pierce a TV or phone cable, and any digging to expose what you think is a benchmark disk should be VERY cautiously done with a hand trowel, not a spade. Look around for warning signs or cable pedestals before you probe or dig.