Signs you'd give if you like someone?

I know that this gonna sound like high school all over again, but oh well. What signs do you girls give if you like someone? There is this girl in one of my university classes and I am obsessing over her. I often catch her starring or looking at me and she just smiles and looks away. Today she just randomly took my pen, smiled and put it back on my desk then by the end of the lecture started to force a conversation with me by asking me a question that the prof answered a second ago. Are those like signs that I need to decipher? Do you think I should talk to her? What should I do or say? There isn't much of a window for talking as everyone leaves as soon as the lecture is over. I am afraid that she might think I am not interested or lose interest herself.
Signs you'd give if you like someone?
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