Joanne,
Well, if I had to know where the burial that accompanied my footstone
was, I would probably begin by mapping other stones (head and foot) as
well as depressions or any other clues as to orientation of other graves.
Then, I would begin removing the sod  from all sides of the stone and
shovel shave a bit to see if and where the soils were
disturbed--suggesting a burial.  I might carry this shallow-depth exposure
to a sufficient dimension to determine the shape of the burial (as indicated
by disturbed soils).  If burials appear both above and below the presumed
footstone, and I had nothing else to direct me as to whether the stone
went with the burial above or the one below, I guess I'd have to stop
there, unless there was a really compelling reason.
What are you doing, anyway?
 
Jane Brown
Geosciences/Anthropology
Western Carolina University
Cullowhee, NC 28723