Indicators of Interest – or 'IOI's' – are the signals a girl will give off or express that hint towards her liking you. These IOI's can be displayed in both verbal and non-verbal form. That said, however, sometimes things aren't always as they seem. What may look like an IOI can sometimes just be an act of friendliness or nervousness. A good example of this would be hair flicking or excessive smiling. While these can be signs of interest from a girl, there are too many possible variables to say with any certainty that these are solid IOI's. After all, some girls just like playing with their hair!
Playful contact initiated by the girl. If you and a girl are getting close and flirtatious, she may playfully hit, bat or push you when teased. This is a good sign that she likes you and is having fun in your company.
If she deliberately knocks into you on the dance floor, she wants your attention. Here you can test the waters by grabbing her hips or pulling her in to dance with her. If she is interested, she'll reciprocate.
She reapplies her make up. Next time you're in the club and a girl you are hitting on goes to the bathroom, take notice of whether or not she has reapplied make-up – usually her lipstick. If she has, she is trying to look her best in your presence, which suggest she is attracted. Pull the trigger and go for the kiss!
Her friend leaves you alone with the girl you are interested in. This is a great sign. Girls know each other inside out, so if her friend disappears and leaves you two alone, you've been given the green light to pull the trigger. Grab a final drink or some food and head back to yours/hers in this situation. Don't hesitate or delay too long.
It's quite common for a girl to mention your girlfriend, even though you haven't mentioned having one. This is her subtle (or not so subtle!) way of finding out your relationship status. Basically she wants to know that you are available as she is attracted to you.
Those are some of the strongest IOI's that you are likely to come across. Other things to look out for are; if the girl is asking lots of questions about you, if she is very compliant to your leading and suggestions or she laughs at all your jokes – even when they aren't particularly funny. No IOI is ever a 100% guarantee of attraction, but keep an eye out for some of the biggies when assessing how interested a girl is in you, and be sure to pull the trigger quickly – hesitation suggests a lack of confidence which will result in a loss of attraction.