[BitWielder1]

My favorite tell is when someone says "To tell you the truth:".
I can't help thinking "What? You've been lying to me up until now? And you probably will continue."
When I get that from a sales guy, that's my cue to leave.

[Blue Ink]

“My favorite tell is when someone says “To tell you the truth:””

Yep. Also variants: “In all honesty...” “To be perfectly honest...”

There could be a nugget of truth in what you’re about to hear... but the speaker is also hiding something. Or lying about something else. Either way, shields up!

[MisterArtery]

My favorite is, “The fact of the matter is....”