The most obvious answer is by referral. A friend or loved one who had a good experience with this process.
I'd suggest you first research the topic itself on your own. If you read a few books on this topic just to get an understanding, you'll know better what to ask during the process itself, and you'll better gauge the attorney's expertise.
The one key thing I'd suggest is that you not go with a generalist. You need to use an attorney who only does estate planning along with the sub topics you seek. There's enough for them to keep on top of in the way of changing laws that you don't want a jack-of-all-trades. Her office may also handle real estate closings (for example) but you want her to be 100% focused. I've had dealings with a full service firm, but each lawyer had a narrow expertise.