Add them as a friend on facebook, and then get into a discussion with them.
Try to psychoanalyze them, point out some holes in their logic and illustrate their hypocrasy. Be sure to use no insulting language, keep the discussion purely logical and focused on the facts.
At this point, they might start claiming you are insulting them. They are losing their cool, showing a sign of weakness. Remember that you should not be using insulting language, but only logical assertions during the discussion, in order for the experiment to be accurate.
Its especially a good sign and even better if they use ad hom attacks on you, or throw desperation slings like trying to discredit you by mentioning your mental illness or your lack of seniority.
Probe deeper. Start making points about their personality, using purely logical claims that they have stated about themselves earlier, and using logic to tie them together. When you hit a soft spot they will claim that you don’t know anything about them.
Continue to remain cool, without using any kind of insults. Tell them that it is possible for someone to know about them, and maybe there was just a misunderstanding with words about it. Remember to keep a polite tone and never to make any insults.
At this point, they will become so frustrated and insecure they will block you, and they will have failed the test.
On the other hand, they could just be sadistic manipulators, and try to block you to test you to see your reaction. If this is the case, steer clear from them, they cannot be trusted at all.