Absolutely. Just talk to her after every priority mission. If you want to lock in a romance with her, be sure to visit her on the Citadel both at Apollo's Bar and later across the Presidium Commons. On your conversation with her while she is overlooking a Presidium balcony, you can mention wanting to be "more than friends" and she will say yes. Then you get a kiss. BAM! Instant asari girlfriend. Good luck!
I...never got the option to romance her. Maybe because I had a ME2 love interest or something? Do you have to be single in order to do it?
It is probable that Shepard must be single. In the Shadow Broker DLC for ME2, when you have romanced another character, she mentions this other person and how it is good that you are there for them. Also, on your import character screen it will show which character you have romanced previously, it is possible this also flags you as "taken" even if you did not play the Shadow Broker DLC.
That's not actually true. In my first playthrough of ME3, when I reached the conversation point with Liara in the Presidium Commons; I had the option to choose 'More than friends'. The Shepard I carried over was in a relationship with Tali from the events of ME2, (So I of course ended up not choosing that option.) so I think it might be a possibility that you missed a conversation flag at some point.
Yes, once again.
I romanced a character in Mass Effect 2 and he well, died in ME3 (most of you who got to this point know who I am talking about), so it got to the point where it asked if I wanted to be more than friends and I ended up kissing and establishing a romance with her as a female character.
Weird. I can't imagine why I didn't get the option while we were talking at the Commons. I don't think I missed any conversations with her (I went to talk to her after pretty much every mission, called her up to my cabin, etc.). In that save file, I was romancing Ashley from ME1 and Jack in ME2. I ended it with Ash pretty early on, but I did cement things with Jack before talking to Liara and got the Paramour achievement for it. Maybe once you're committed to one romance the flags for the others turn off? A la Dragon Age II?
Despite what the IGN wiki says, you do not get the option to romance Liara the first time she comes to your room (this might be different if you chose her in ME1). You don't get the "more than friends" option until you talk to her on the Citadel a second time. This occurs after you complete a priority (or possibly side) mission, following Citadel 2. You'll get an email telling you to meet her on the commons. It is there you get the option.
BTW, solidifying things with Jack or Garrus will shut down any other romantic possibilities.