Aside from research to find records of an old cemetery, the dates on the headstone will yield some answers.

If the dates are from the 1900s you may be able to probe the grounds with a strong metal bar. You use it as a probe and see if you hit any hard material such as wood or concrete. That should tell you whether someone is there.

If it's very old, you probably will not be able to find evidence. Generally speaking, if a headstone is installed, unless it was just disposed of, there is a reasonable chance it’s and actual grave.