There are a few things you should look for:
1. Make sure that it is attached to the glass, a rock or something in the tank. An anemone that is not firmly attached is not a good sign.
2. The mouth should be held tightly closed.
3. Although a freshly shipped anemone may not feed right away, it never hurts to bring a small piece of shrimp with you and ask them to try and feed the anemone you want. It may not eat it, but it should at least be able to hold the shrimp for a few moments. If it eats that's even better. (I know that sounds funny, but I have actually brought pieces of shrimp to my LFS)
4. Once you have made your choice, when they start to take it off the glass or rock, the anemone should contract violently to try to get a better hold. If it doesn't, tell them to never mind and that you will come back tomorrow to see how they are doing.
Do not disregard any of these things because one is prettier than the others. It is very important to be able to say no to all of them if they don't meet the criteria.
One more thing, a BTA will tend to expand bigger in your tank than it was in the LFS, but if you have a choice between two that are equally healthy, get the bigger one. Larger anemones tend to be more attactive to clowns and they can also better withstand the beating some clowns can give to them.