How do I know if she's interested?

[Matt]

I really like this girl but I can't tell if she is into me or not. We were at this party together and to start the night off she came up and gave me this huge, lingering hug. I would say it lasted at least 30 seconds. Later in the evening we were in the basement together and a slower song came on (Angel by Shaggy), she embraced me, started swaying back and forth like she wanted to slow dance. This lasted about a minute until the DJ changed the song to Make it Nasty by Tyga and she walked away. About 20 minutes after this she found me again right when a faster paced song came on, she pushed me against the wall and started grinding on me. A few days after this I learned via Twitter that she loves when guys play hard to get. The times I text her and we talk for awhile she seems genuinely interested in the conversation but we have been to a few parties together since this night and she hasn't seemed as interested. Based upon that night at the party, do you think she may be interested in me? I don't want to overstep any boundaries. Also, how can I "play hard to get" just enough so she knows I'm interested but not too interested?