It’s in the way they look at you, want to be around you and want you to know they find your interesting. They make real effort to get to know you as a person – ask personal questions about you, your work, your family or your day, and remembers things you’ve told them about you.
The one who is not interested may keep in contact but asks no personal questions about you and avoids any personal questions about themselves. If he or she is lonely and just enjoying the attention, you find yourself in the role of friend/social support or therapist. Once they find someone else to lean on, they move on without any guilt or shame about using you.
But even for someone who is interested, if you make it too hard to reach you, to get to know you or spend time with you, the interest will fade out very quickly.
In other words, don’t be too elusive, too closed off and too busy that the other person assumes you’re not interested.