What should I do, Im confused and need guidance, Please help!!!?

Ive been seeing this girl in my office for the past 2 months and am interested in her. I initially started off this conversation and she too responded . But she sometimes seems to ignore me for reasons I dont even know. Sometimes I need to catch her attention in office for even a Hi!!!. I know she looks at me sometimes and looks away when I catch her staring but am a still confused. The other day I had invited her and her friends for lunch for my treat and she was polite enough to say that she had made other plans . recently I approached again and fixed up a dinner meeting and she agreed to come . She eventually came too. It so happens that she is married but does not stay with her husband.She said that she hasnt seen him for years. She was open about it but she says that they are still in touch which is very confusing. I like her and want to take her as my companion but am not very sure if she thinks the same way. she has been noticing me and one day she even remembered the dress I wore to office but is to herself most of the time . I infact asked her why she is like that and she said that she was always like that engrossed in work. I got her phone number but did not want to show desperation in calling her and she has not also called me. Need some help to take it forward and win her.. Im true but I dont know if she likes me just as a friend or more than that..It was her bday last week and I had gifted her a dress and she wore it the next day and stopped and showed me that she was wearing one. I smsed her saying that she looked great but did not get any reply . She however talks normally and has not avoided me after that .. what should I do... how do i know if she likes me and is interested in me?