Recently a neighbor described a "slave cemetery" she'd found. When asked how she had identified it as a slave cemetery, she said the stones were (all but one or two) unmarked. This belief is totally anachronistic. Blank stones, head and foot stones with no inscriptions are not the earmarks solely of slave graves in the South. First, illiteracy was the norm for all three prevalent races in the antebellum South, enslaved and free and, second, some religions disapproved of the use of grave marking.
Even amongst sects that took pride in their literacy, inscribed stones could be controversial. Varieties of Quakers and Baptists, alike, have gone through moments of discomfort with grave markings. Many genealogy sites on the internet carry a warning or observation along these lines: "There was a period when Quakers were discouraged from marking their graves. An old Quaker Burying Ground may look as if it is only partially filled when, in fact, there are many graves that simply have no stones." What held true for the Quakers, particularly in the 18th century, held equally true for some Baptists. So, an unmarked stone is no clear earmark of anything, not even of economic standing.
Even within the vague ambit of race exclusive graveyards, it is nearly impossible to discern differences between slave graves and the graves of freed slaves, or those of other people of color; Native Americans, and multi-racial folk. Again, particularly in the case of colonial era graves and graves of the early republic it is all a matter of temporal and physical context.
Before asserting conclusions about the social and economic status, legal status of the residents of a graveyard, take the time to research the deed and grant records to ascertain if the land was associated with a church at any time. See if it was part of a plantation. But don't assume that if there is no documented use as a church site that it wasn't as the documentary record is very, very incomplete.
Before there were "plantations" in much of the south, there were communities of squatters, so graveyards can predate legal land records. In these cases, you may have to suspend judgment about the age of the graveyard and the makeup of its residents until after a careful archaeological analysis has been performed.
Before investing in archaeology, though, consider that graveyards are, by definition, sacred land. Archaeologists regularly invade sacred space to study burial procedures, grave goods, and so forth, but even for them it is becoming more and more difficult to rationalize disturbing final resting places. If we are unable to determine the racial or socio-economic identity of a corpse is it all that important? Probably not.
It should be sufficient to know that a graveyard exists. Mapping the landscape remnants of the graveyard; grave locations and alignments, head-stone and foot-stone locations, boundaries, and so forth should provide all the information one can reasonably extract from the earth. Mapping plus documentary research will tell virtually everything the present needs to know about the occupants. It is, in fact these spatial issues that are most informative, so, whatever you do, locate and map our graveyards for it is one of the most effective ways for us to get some understanding of the location and concentration of people in our common past.
Try to set aside anachronistic prejudices about the quality of a gravestones relationship to the quality of the grave's content. In and of themselves, unmarked gravestones can tell us practically nothing whatsoever except that 'neath those stones lie the remains of somebody once loved and missed and more or less tenderly laid to rest.