Actually there are DNA tests that will show various nationality strands for people.
Guide to Ethnic DNA Testing: How to Prove Your Ancestors? Ethnicity
Many people of African American decent have been using this to try and determine what part of Africa they are from.
Also there is really no such thing as a full blood Haitian. The first natives of the land were almost completely wiped out by the spanish. The ones not killed in war died of the diease they were not able to control. There were only a few that escaped to the mountains. Those almost all breed with escaped African slaves. So it would be almost impossible to be a full blood Haitian. This is going back to the 1500's.
If you mean from the main population of the mixed Spanish, African and Haitian tribes.