Most states require some kind of licensing these days, so if yours does be sure your MT has it. NCTMB is a national certification but some states use their test for passing the licensing exam, which would mean that everyone in that state had it. AMTA and ABMP are national associations but it is not required to have those designations, so some good therapists do and some don't. Most places will not hire an MT without professional insurance but it's a good question to ask.
I used to belong to several associations but now I'm semi-retired and they are expensive, so I dropped them. Word of mouth is a great source, so ask around for a good MT. When you make the appointment ask for someone who has experience in what you need or want. For instance, deep tissue, stone, maternity, child, etc. If you just want to relax any MT can do that, it's basic massage. If it's your first massage a good MT will educate you about what to do, what it's like, etc. and take a short medical history. If you go back to the same MT, they will keep a record of your progress. These are signs of a great MT, although some establishments, and some good MT's don't follow this procedure, it's a good idea to do so.
