I guess you might be able to find reviews online. Another thing you can do is go in there and talk to the tailor, find out where he trained, his particular style etc., maybe ask a few other questions.
A good question would be to ask about whether the jacket is full, or half-canvassed. If it's only half, that might be a warning sign.
These are just my ideas, unfortunately i'm not earning enough yet to be able to afford a tailored suit.
In that case, it would be more difficult to ascertain but then I suppose they would have the suits made centrally, so it would be more likely to have reviews online.
Canvas is what's inside of a high quality suit jacket. It helps it keep its shape but a lot of cheap off the peg suits won't have canvas in anymore.
This article is pretty good:
Ask for references, people he/she has done work for before. If they are any good they should have plenty and be willing to provide them; if they don't then they aren't any good and you shouldn't use them.