Mate, you're always taking a punt, the only way to find out if they're any good is to try them out. There are certain things you can look for like the quality of their handwork, such as asking if they do any handfinishing and asking to see their buttonholes for example, but there are only indicators. There's no point in lots of handwork on a garment that doesn't fit at all.
Here you'll find some posters saying that it takes a couple or even three iterations to get a really great fit and I would tend to agree with that, it takes time to know each other and work out the tailors capabilities and for him to learn about you.
Thanks, I think I get what you mean now. So it's less about the absolute skill of the tailor as it is as much about service they offer: how well they communicate and listen to their clients?