There have been studies of the Bubonic plague of 1666 that show there were also some people who never contracted the plague despite being in direct contact with plague victims. Tracing their descendents it was found they had a unique genetic marker that apparently gave them immunity. That said I would not want to find out if I had a similar immunity to Ebola.
If a genetic marker that confers natural immunity to Ebola is identified, you don’t need to be exposed to Ebola to see if you have the marker. A simple genetic test can show if the marker is present.