A person infected with Ebola can’t spread the
disease until symptoms appear
The time from exposure to when signs or symptoms of the disease appear
(the incubation period) is 2 to 21 days, but the average time is 8 to 10 days.
Signs of Ebola include fever (higher than 101.5°F) and symptoms like severe
headache, muscle pain, vomiting, diarrhea, stomach pain, or unexplained
bleeding or bruising. (source: http://www.cdc.gov/vhf/ebola/pdf/what-need-to-know-ebola.pdf)
Diagnosing Ebola in an person who has been infected for only a few days is difficult, because the early symptoms, such as fever, are nonspecific to Ebola infection and are seen often in patients with more commonly occurring diseases, such as malaria and typhoid fever. (source: http://www.cdc.gov/vhf/ebola/diagnosis/index.html)
They go onto list specific tests that are available for use. So, only after the onset of symptoms can they test you for Ebola. I said something earlier about a 7-21 day window. Looks like I was wrong, it's 2-21 days.