Welcome!
At that age, you mostly just need to follow the child's lead, as you are already doing.
Parenting a GT preschooler is mostly just like parenting any other child, only earlier and more of it!
Generally speaking, I don't think testing at 2.5 is going to tell you too much that's useful--a lot can happen before she hits "real" school. Unless you have a specific question that you need to have answered right now, it would probably just be an expensive confirmation of what you already know: she's smart! Well, duh! You'd probably just have to test again in a few years. And besides, you probably don't really need a test right now; a mom usually knows.
I think it's safe to say that for now, you should trust your gut: if you think she's GT, she probably is, and you should approach her accordingly. If other parents don't believe you when you talk about what your child did or said yesterday and you have to censor what you share at playgroup meetings, there's a good chance your child is GT.
The best advice I can give you is to treat her as a whole person, not a little brain on legs. (Not that you are doing that, of course! But it can be easy with these kids to forget that they're in many ways kids like any other.) Most of what kids learn at this age--and that's ALL kids, GT or not!--is through playing. I mean, if she WANTS to do workbooks and read science books aloud to you or something very school-like, then follow her lead. But if she wants to ride her trike or play soccer or dress up dolls, then that's good, too. None of that is a "waste" of her time. She has her whole life for sitting down and studying; now is the time for her to play!
With that said, a preschool that suits her needs--whatever those needs are right now--might be a good idea. Just don't assume that a highly academic preschool is the best choice. It may be, if their curriculum meets her intellectual needs. But it may not be if they're too far behind her (or ahead of her, of course, but for GT kids, that's not usually the problem).
Many (not all!) of us have had good luck with non-academic, half-day pre-K programs. The kids get the social time and the fingerpainting mess for someone else to clean up (don't underestimate the appeal of THAT!
), and they can get their intellectual needs met at home during the other half of the day. It works surprisingly well a lot of the time.
(In the interest of full disclosure, my mom is a retired pre-K teacher, so I think pre-K can be a really good thing, given the right match between child and program/teacher.)
Another tip that I wish I'd known earlier in the process: expect to reevaluate the school situation as often as once every 6-12 months. Things change fast with these kids, and what works today may not work tomorrow. Many of us have found that we no sooner have a solution in place that seems to work, but the child has outgrown it.
Until she enters pre-K, just keep doing what you're doing. Often you do have to use materials for older kids. That's fine. Just be sure that if she's emotionally sensitive (as many GT kids are) that nothing in the material will be troubling to her. Otherwise, just keep following her lead, following her interests. Answer her questions. Go to the library. Read to her about things she loves and topics she knows nothing about yet. Go to the park and the museum and the zoo. Make friends. Swing.
You know, be her mom!
There's plenty of time to be stressed out later. Trust me!
For now, just have fun with her. Happily, there's nothing wasted in time spent doing that.
