Until you officially "lock-in" a romance in ME3 you can flirt with as many people as you want.
Liara joins you early on but Tali doesn't come back until about halfway through. When you meet Tali again you have a chance to invite her up to your cabin. During this conversation you can lock-in her romance. For Liara she will eventually send you an e-mail asking you to meet her on the Presidium. You'll find her leaning against a banister on a balcony. During this conversation you can lock her in.