The easiest part online is checking their qualifications. References and results are easy.
The hard part is you really need to have a chat with them and see if there is a fit. Does their coaching style work for you? There are pushy coaches, indifferent coaches, whiny coaches, polite coaches, coaches who will call you names. Do their diet, training, supplementation philosophies align with yours or can you put yours aside and follow their directions.
The last thing you want to do is head in two different directions or be in conflict with your coach. Many, many people fail because they won't follow and they won't lead so they end up half way to nowhere.
Are you coachable? Seriously, Are you willing to be coached or are you just looking for a cheerleader?
There are some obscenely qualified coaches here and in places like this. Finding one who you can succeed with will take a little leg work if you're serious.