First impression!
Yes, that’s the most important. The first impression is either good or bad. Trust your gut feeling.
Any references?
A good tailor has references as well as returning customers that also means experience, which is very important. So if your tailor has lots of referrals, it is a good indicator that you are in the right place.
Quality of the details!
The devil’s in the details, especially for a suit. Tiny little details make a suit special, which are only visible when watched carefully. What are these? The quality of the hand sewing, the regularity of hand-stitching, the uniformity of the dressings, the consistency of the seams, etc.)