A good way to tell is to start with light touching, brush her arm and hand. Judge her reaction, does she reciprocate or does she pull back. Gradually move up the level of touching , i.e leaving your hand on her arm or if sitting on her knee or thigh. Keep the level increasing, move closer, if she is still receptive then I would start flirting, lower your voice or whisper things to her, get her to come closer, etc.
That is creepy.
There is no way to tell by regular social interaction. You need to escalate your intentions, personally I prefer to do it gradually as outlined above rather then go all in and ask her out. It could be awkward going forward since you work together if she said no. Obviously you will need to modify above as this is in the work place. Try the light touching to see how she responds, keep it natural. If that goes well then ask her out.. Lunch is a safe bet with a coworker.
She ll keep talking to u and keep the conversation rolling
Read her body language, it's usually quite obvious if she's into you or not.
Ask her out and find out
propose to her!