Slaves were listed on the Census around 1850. The US had two types of Census records at that time. The first was a schedule 1 where it listed all free persons and the second is a schedule 2, where slaves were listed. If you find any relatives on the schedule 2, they were slaves at that time. There might be other ways to tell if your ancestor was a slave or not but this is one I know about. I hope this helps you out.