First of all, you can never be sure about the quality of the massage you will receive until after you have received it. Even at the same place, different masseuses will give you slightly different massages, varying in both technique and "quality".
That being said, I get that you're looking for legitimate no-nonsense massage shops. Some of the most obvious indicators to me are
Obviously, you should avoid the typical red light areas, such as Nana Plaza, Sukhumvit Soi 22, 24, 26, and so on. Granted, there are some legitimate places in most areas, but as a novice you're better off seeking out an area where your chances of getting a good quality massage are higher.
Usually the "extra service" places will be open later than 10pm, and many (most?) legitimate ones will close about 10-11 pm.
Do some searches on Google Maps in an area you're interested in, and you should be able to find reviews, photos, website links, etc. so you can make a good assessment if the place is legit, has good reviews, etc.
Most normal massage places require their masseuses to wear uniforms, often consisting of a short sleeved loose-fitting shirt and a pair of loose-fitting pants. The "extra services" places might also use uniforms, but trust me, you'll know the difference. If there are no uniforms, it could be either way, but it's a gamble.
Obviously, the "extra services" places will typically employ younger and more attractive masseuses, and they will often wear more makeup and so on. If they call out "massage" when you walk past, I would keep walking. Many of the normal massage places have a small seating area outside where the masseuses will sometimes hang out, but rarely will they shout at you when you're walking past them.
Most good quality normal massage places will have a nice atmosphere inside with some decorations. I often find that the places that have an untidy or disorganized look & feel already when stepping inside the door will usually not have as good massages either. This is of course not always true, but more of a general rule of thumb. Besides, to me it's more relaxing to have a massage in a nice, quiet, clean and cosy environment than in a disorganized, loud or unclean one.