From decades ago when looking for family headstones on my mother's side of the family,
I've found headstones (lying flat) under 6 inches of soil or greater (burials in the 1800's) by using an old wooden cane, metal rod (be careful) or you can use a fiberglass rod found in stores like Home Depot, Lowe's etc that mark driveways or fit in snowplows.You can also use a wooden dowel rod.You may have to modify the prior options to make it work for your technique. Soil types may make the level of difficulty greater in some places than others.You can also carry a few gallons of water with you to soften up the soil. If you know where to look or how a cemetery was laid out this can also be helpful. Traditional burial methods in the time frame (1800's and so on). If you know what you are doing and are careful you can use a shovel too to find them. I've found and realigned foundation stones and granite headstones flipped over and buried down in the dirt. Hogs got in and damaged cemetery.A good lever bar (metal) or an old barbell bar can be used for a lever bar too.You may need a stone or concrete block to use with the lever bar.
Divining rods and/or ground penetrating radar are options in finding graves also. The ground penetrating radar of course can be expensive.