I think determining whether they are or are not is very hard and full issues like bias toward english proficiency, culture and a whole bunch of stuff that could be problematic to screen for.

Thats why I phrased the question as assuming they are a foreign agent because the due dilligence one must do is probably particular to the job and company.

The evidence however seems like it will always be more or less circumstantial unless you have the time and resources to devote to a thorough background check.