A sharp intake of breath is a pretty solid indicator.  The more pronounced it is, the more agitated they are that you're still talking. (Unless they're just yawning.)<p>Looking away for a while and suddenly looking back with their mouth open slightly can also be an indicator.  If they have their mouths closed tightly, they're deciding whether to interject, so you might want to check a sentence or two later to see if they've made up their minds.