Great question - perhaps the answer's different for each couple.
Start by asking your friends, and your married friends' parents: word of mouth is a great recommendation!
Similarly, your venue might have a list of trusted providers, but don't go by that alone and be aware that venues shouldn't 'bundle' celebrants in with their services or make specific recommendations, as couples should be free to choose someone who suits them.
Then to the internet. And that's a job you can make easy or hard, perhaps depending on your skills and/or your time! Do you judge a book by its cover? It's an unavoidable starting point... We're all drawn to people we find attractive, but search a bit deeper then for clues about experience, quality training and standards, and something that gives you a sense you'll 'click' personality wise!
Your celebrant is the pivotal person who kicks off your big day! Search for someone who instils confidence. Some couples underestimate the importance of their celebrant but it's worth remembering that you're also preparing for your marriage at the same time you're preparing a celebration, so choose someone who appears to have a balance of maturity and legal expertise, as well as a style that might match yours and importantly, a sense of theatre!
Take the time to find the celebrant's Facebook Page. You might find a variety of interesting, fun, helpful or thoughtful posts over a period of time... not just photos of weddings, as many couples don't want to share their weddings publicly.
And finally, you can search on the association websites. It's reassuring to know your celebrant values the benefits of membership, and will have the necessary insurance cover and network should anything accidentally go wrong!
Then contact two or three. Don't just ask for a quote! Try to ask something that draws a bit more from their response, and that might help you narrow it down a bit more!
You've found me here... how about a chat? You'll hear my voice and that's a very good indicator of whether I might be the right celebrant for you!