There's several signs...
the first is MEGO (or My eyes glaze over); they lose concentration and focus, sort of like people watching a Powerpoint presentation...
Second are interruptions. Usually these are using 'termination phrases', things like "uh, yeah, that's nice', "Look at the time', "I gotta go', and others. If they start gazing at other things than you, that's another.
If that's what you're talking about.