The Little Things
A good sign of your co-worker acting flirtatious with you is if he seems to be paying a lot of attention to you. When he steps outside to get a hot beverage, he always asks you if you want anything as well. He is excited when something goes well for you and offers his shoulder and listening ear whenever you want to complain about something. When he comes back from lunch, he brings you a pastry. These are all signs that you are at least a little special to him.
Hanging Around Your Desk
If you notice that your colleague comes by your desk at the beginning of the day, just to say good morning, this is a good sign she is flirting with you. She will also stand around your desk, asking you questions about what you did the day before, what your plans are for the weekend, and what you think about one thing or another. If she frequently stops by your desk to chat, it means that she likes being around you.
Prolonged gazes, cheeky smiles and accidental touches are signs of flirting in any environment. If you are receiving secret looks that last for just a little bit too long, or if your colleague seems to be all smiles whenever you're around him, and if he uses any chance he gets to touch you, he is most definitely flirting.
Waiting for You
At the end of the day, your colleague lingers around the office, waiting for you to get ready so you leave together. If you are having drinks with co-workers, she leaves the party when you do and maybe even walks with you a portion of your way. She might eventually even ask you if you want to have a casual drink after hours or suggest you grab lunch together.