It's going to be a little hard since you said that you don't have a poker face but your best bet is to just see how she interacts with you compared to others.
Does she laugh at a lot of your jokes? Does she spend a lot more time with you than others? Does she stand close to you or make physical contact? You have to notice these kind of things.
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Does she laugh at a lot of your jokes? Does she spend a lot more time with you than others? Does she stand close to you or make physical contact? You have to notice these kind of things.