I was also wondering: What are the features/signs that make a shirt unmistakably bespoke/MTM? This is asked from the perspective of someone who would like to be able to identify bespoke/MTM shirts when others wear them, and who would also like to order his bespoke/MTM shirts accordingly.
you cannot tell by the quality because there are many quality rtw shirts like kiton, borrelli, etc... the tell tale signs are proper sleeve length, proper cuff size, and the shoulder seams should be on your shoulders. bagginess in the waist is very commom with rtw shirts. also, if you see a shirt that has two buttons on the cuff for adjusting the cuff size, that's obviously not a custom shirt.
To me, it's just common sense that you can't use your skin measurement and assume that is the exact length your sleeves should be.
actually, you can use your skin measurement but you need to measure down to the middle of the back of your hand. also, if you are measured properly, one side should be slightly longer than the other by at least 1/4". everyone has one arm longer than the other.