1. Scratching//face touching of the brow, wrist or cheek. This is hard-wired response and it tells you that she is thinking of having sex with you.
  2. Proximity - shortly following the previous IOI is proximity. The girl will either stand next to you for a bit so you can talk to her or leer at you from a distance.

I had this done to me today by a hot girl that was working out at a strength camp. I stepped out of my car and several women looked at me. The hottest girl caught my eye and started to scratch her eyebrow shortly after and then come stand about 3-5ft away. Her body was amazing looking and had nice abs. She even stopped what she was doing as well just so she so I could approach her. Girls check guys out all the time! You just aren't seeing it. This is how you know when they are doing it. If you see them do this then go introduce yourself and start building attraction/comfort.