I've reported Master Tathagata's definition that Enlightenment is 'the opening of one's eyes to the world'. He's also said that Supreme Enlightenment (that of a Tathagata) is opening eyes to reality completely. So the first thing is - ask them any questions in reality - if they are unable to give the answer that can be verified in the reality (if they cannot realize what is in reality) then they have not completed their Enlightenment.
A second thing I'll leave with you is like this. A bodhisattva cannot have attained Enlightenment completely - because if s/he had, then the one would be a Buddha.Yet it seems that a Buddha only comes to the world once every 3,000 years or so. The term of Bodhisattva means "gods who have good power and are seeking for the truth". (Sorry cannot cite as it was in yet-untranscribed audio from Tathagata) They protect Buddha, but could not remove their karma totally yet, so they are still seekers. However, sometimes some Bodhisattvas who have a power to stop their karma can get into the same dimension that the Buddhas reside in (paradise). So the second thing to check is whether they claim they are Enlightened. It's a red flag to me.
The third thing is that an Enlightened Being never lies. Instead, they bet their life every day in order to inform people of the truth - this is the proof of their love for mankind, and it is the greatest teaching. So your continuous verification of what people say (imho, according to the facts in the world) is essential whether or not the person actually has evidence that they know truth, or is telling a lie.
Hope this helps