trump's tell? When he includes "sir"
Of course I'm very tongue in cheek about this as it could simply be spurious correlation given how much and how often he lies.
I've fact-checked every word Trump has uttered since his inauguration. I can tell you that if this President relays an anecdote in which he has someone referring to him as "sir," then some major component of the anecdote is very likely to be wrong.
Lots of people do call Trump "sir," of course. But the word seems to pop into his head more frequently when he is inventing or exaggerating a conversation than when he is faithfully relaying one. A "sir" is a flashing red light that he is speaking from his imagination rather than his memory.
In poker parlance, it's a tell.