If the head is tilted slightly and one ear turned towards you then this is a good sign that they are listening to your words intently and are taking in what you are saying.
If you also notice some people are tugging or touching their ear lobe, it is a sure sign they want to hear more of what you are saying. Try to distinguish whether they have an itchy ear or do want to know more. Typically if they are scratching their ear it will last no more than a second or two.
If you notice members of your audience place something in their mouths like a pen or their glasses then this is a good indication to you that they want to know more. Think of it as them wanting you to feed them more of what you are telling them.