Upon noticing one has received a text, the hypothesis one makes regarding who it was that just texted them, prior to checking the phone for confirmation or contradiction.
It only counts if the message guessed upon is the first of a conversation.
John: "Man, I bet that's Ryan texting me."
Evan: "Really? Why?"
John: "I was talking to him before today about hanging out."
John: *checks phone*
Evan: "Was it him?"
Evan: "Nice text-guess, man."
Alex (in thought): "That better not be Jena texting me.
Damn it, I know it's going to be her."
Alex: *checks phone*
Alex: "AH, SHIT!"
James: "What's up?"
Alex: "It was Jena, again. I text-guessed it and everything, too."