I think people are usually pretty obvious when they show that they're not interested. But when they're not yawning/ turning away/ fidgeting/ looking at their watch/ seeming distracted/ seeming bored/ saying too little (for them) or otherwise show clearly that they're not interested, I can never tell if they are interested or just hiding that they're not.
Of course, that's assuming I even have something to say, which more often than not, I do not.