My first initial thought is the cost of therapy. Since I went the online therapy route, I had 3 choices and tried to go with the cheapest options first. However, I couldn't get in contact with them so I had to go to the more expensive therapist. I ended up liking her a lot, but before I proceeded, I tried my best to do a more extensive "scoping". The biggest thing for me was word of mouth. It was hard for me find any reviews for her, but eventually I found one person who went to her and enjoyed the experience. That was enough for me and I strongly recommend you finding reviews from other people or asking about their experience with them. That alone could give you a peace of mind. For me, the therapist having experience with other trans people (as well as writing letters of recommendation) was very important.
As for therapist, I liked her because she was funny, blunt, and to the point. She also didn't act as a "gatekeeper" After two sessions, she knew that I was serious about transition plans and top surgery, so shes going to write my L.O.R at my next session when I have the funds, which I think is fair for my situation. Even though she doesn't string me along, she still cares about my well being that isn't limited to just a top surgery letter, and I think that would be a good trait to look for in a gender therapist. That's where word of mouth/reviews come in. The only thing I disliked about my experience was the price. For various reasons, I wasn't able to get it covered by insurance so I had to pay out of pocket and as an unemployed broke college student at the time, that was hard on my wallet, but as far as the service itself, it was excellent.
When also looking for a gender therapist, make sure you check to see that they are accredited and properly licensed. Don't be afraid to ask them to show their credentials.
Best of luck!