Example of reactions to our actions could be the look we give someone across the room or perhaps the look we didn't give. We might stare at someone at a distant before long they are returning that stare.
Once in a very crowded room I felt I was being watch and yet I pretended it was all in my mind, we have to be careful with this as we could make a fool of ourselves and think that a person might like us and go upto them (depending if we are bold or not) only for them to tell us we have a piece of food on the side of our mouth.
Really its the sequence of actions we look for in this case...
Whats behind the smile
To they stare back for more than 2 secs and smile
Is their body pointing at you or on the side
The obvious of all would be if they waved at you, winked at you or raised their eyebrows.
I keep reminding myself that look for more than one sign.