Close the Loop
You can tell when someone isn't paying attention. When they are distracted while you're in their company. You can sense a shift. Something is different about them. Something changes in how you are around them.
There was a time when it was easier to tell whether the distraction was something major (a loved one's illness or death, financial burdens, etc.), something less so (waiting on an important response, daily work, etc.), or something minor ("I just posted this picture online and am wondering what the response is").
Today, our level of distraction is high. How many ongoing conversation are you currently having? I have 5, open ended, never finished, conversations that take some attention away from whatever I'm doing right now.
Close the loop. Say, "until next time," or "to be continued.” Hi-five at the end of a conversation. Send a smilie face. That way everyone knows the engagement for the time being is done, finished, complete. Your full attention may turn towards the next engagement at hand.
If you can tell when someone else is not fully there, they can do the same with you. And you know how the I'm-here-but-I'm-really-not vibe makes you feel. Let's not be that person. Let's be present, and if it's not possible, let's be honest about it. That'll help us all realize what it is we're actually caring about.