Although there are no foolproof ways of knowing whether your therapist is an expert or a wannabe until they lay their hands on you, there are some things you can do to lower the risk of leaving a treatment with nothing having been thoroughly massaged except your wallet.
There are so many different accredited qualifications in this country that listing them all looks like a small mammal has scurried across your keyboard. Probably the two most recognised awarding bodies to look out for are ITEC and VTCT, but other acronyms you might see flung about in course titles include NVQ, BTEC, MTI, CIBTAC, VRQ, TSHB, AOR, C&G, SCQF, QCF, IIHHT, ASL, APNT, FHT, IPTI... it's an alphabetical minefield, and one that for the most part is best ignored. A Level 3 qualification is the industry standard, although if you want a good sports massage, Level 5 is your best bet.
The bad news is that duration and quality varies wildly from training provider to training provider. For example, a quick browse on hotcourses.com shows that the same ITEC Level 3 Lymphatic Drainage course could take two days to complete at one school, and 3 months part time at another. Both therapists would look exactly the same on paper and get identical insurance cover for their work. The only way to really find out the truth is to ask for the name of the school they attended and do some online detective work yourself. Please, PLEASE don't end up with a therapist with only a day or two of training. There's a chance you could strike lucky and find someone naturally talented who you're happy with, but then again there's a chance you could win the lottery. Not great odds, basically.
Insurance is an absolute must. I can't say it enough - do not go to an uninsured therapist. Massage is a very safe therapy, but accidents happen. What if your therapist unknowingly uses a nut-based oil when you are allergic to nuts, or their massage table decides to give up the ghost whilst you're on it? Insurance gives both you and the therapist the peace of mind that you will be covered and compensated for, no matter what.
If they are a member of a professional association such as CThA or FHT, great - it's no guarantee of competency but at least means they take their profession seriously enough to take an active part in it. All professional associations require a yearly payment from their members - therefore an active member (hopefully) indicates an active therapist.
Another obvious thing to look for is experience. How long has your therapist been massaging for? This is difficult to prove, and most of the time you will have to take them at their word. However if you do want to check, ask to see their first massage certificate, which will include the year that they qualified. Although not foolproof (they might not have started working professionally until some time after graduating) it will at least give you an idea of how long they have been interested in massage for as a career. Although it's possible an MT could be giving crap massages for years, it will usually be the good ones who stick around in this profession the longest.
Short taster sessions (perhaps 20 or 30 minutes in duration) can be incredibly useful, for obvious reasons. If your potential therapist does not actively offer these, ask. Most of us are happy for the opportunity to impress a potentially life-long client, and it can give you all the information you need about whether they're a right fit for you in one economical, bite-size session. Whilst you're there, ask yourself some questions: Is the treatment room tidy and uncluttered? Does your therapist look neat and professional? Are they responsive to your needs and requests, and seem focussed on you throughout the session? Does your treatment start and end when it's supposed to? Do you like the therapist? These can all help you decide whether it's worth returning for a second visit.
This one may be controversial, but in my experience, if you want a really good massage, avoid spas. I'm sure not all spa massage is bad. Perhaps I've just been unlucky. However I do know of many high-end spas that pay their therapists little more than minimum wage, making them work on commission with long hours and few breaks. Unsurprisingly, the best ones don't tend to stick around for too long. If you want something more than just oil rubbed into your back, go to a clinic where they specialise in bodywork, rather than beauty.
Give them a call. A good MT will be happy to answer any questions you might have, and you can get a real sense of how they work as a therapist from listening to them speak. Are they interested in finding out the reasons why you're seeking treatment? Do they listen and seem to care about what you have to say? Do they instil you with a sense of confidence? If you leave a voicemail, do they get back to you promptly?
Just like the gym, if you want to go regularly for massage, choose someone local. Nothing kills a post-treatment buzz quite like having someones armpit in your face on a crowded tube. If they're close to your place of work, perfect. If you can walk home afterwards, even better. Recommendations are also good. Ask people who are interested in health and well-being, such as gym instructors or yoga bunnies, if they know any great local therapists. There's a good chance that someone who takes an active interest in their own body will have at least a couple of numbers up their sleeve.
Lastly. Websites aren't everything, but if they do have a website, check out the language they use. Not every MT has a flair for design or an in-depth knowledge of HTML, but a good one should be able to convey a sense of being knowledgable, without alienating their clientele with complicated terms and niche phrases. It's very easy to cut and paste from an article about the long posterior interosseous ligament and the sacrococcygeal plexus - but if they can't phrase things in a way that that's simple and easy to understand, then there's a fair chance they don't really know what they're on about either.
I can't promise that following these tips will lead you to a great massage therapist - nothing but actual hands-on time can decide that. But I hope that in the gamble of choosing a therapist with whom you can have a wonderful relationship with for years to come, using these points as a guide might help to raise the odds a bit. Good luck!