Think a bit if you were the one screening potential traitors. There are two most obvious threads to investigate. 1. People who would work for allies (considering it a lesser sin). 2. Ethnicity. E.g. Jew by "Israeli" agents, Chinese by "Chinese" agents.
Egypt says verdict in Al-Jazeera case expected Saturday (AP)
from the article: U.S. arrests ex-NASA scientist over bid to spy for Israel