'You'd be a great father' how do you know?

When you have those initial conversions with people, the subject of marriage, children etc comes along.

Person X: 'How long have you been married for?'
Bad: '5 years now'
Person X: 'How many children?'
Bad: 'none that I know of'
Person X: (after they get the joke) 'It' ll happen, and I think you'll make a great father'
Bad: 'Really, how do you know? '
Person X: ' You can just tell.'

Now I've had a very similar conversion a number of times, and it's pretty much the same. I just don't get how anyone can predict a man's potential of quality fathering pre well fatherhood.

Do I give off some awesome paternal aura? Are they just plain lying, or am I truly an awesome daddy in the making.