In an excerpt from his book 'Gentleman - A Timeless Fashion', Bernhard Roetzel elaborates on the seven major things to look out for when buying a new gentleman's shirt.

Firstly, a good shirt has removable collar bones where required by the shape of the collar. This is particularly the case with turndown and cutaway collars, but collar bones would be out of place in the soft collar of a button-down shirt. Most collar bones are made of plastic, but some gentlemen's outfitters offer brass collar bones as well. Whatever they are made of, they give the collar the right curve and prevent the collar tips turning up. 

Secondly, the split yoke, which origins lie in traditional shirt making. As people generally have shoulders of different heights, a split yoke can be used to adjust the fit of a shut precisely to the customer's stature. On ready-to-wear shirts the split yoke is just a detail which suggests more expensive work; it is a costly detail, though, because every additional seam is a not inconsiderable cost factor when shirts are produced in large numbers. Such details may be unimportant for many. 

Thirdly, patterns are only matched exactly on very good ready-to-wear shirts, though this is always done on custom-made shirts.  For example, stripes or checks should match exactly where the shoulder joins the sleeve. Raised (or "French") seams are used on those parts of the shirt that are subject to particularly hard wear. For a raised seam the two pieces of material are sewn together, turned over, and sewn again. This procedure is expensive, but ensures that the seam will be durable.

Fourthly, traditional buttons made of mother-of-pearl are obligatory features of a good shirt. Mother-of-pearl buttons are so hard that they will break the needle of a sewing machine. The button facing used to be an additional piece of material sewn onto the shirt, but today it is made by folding over the material at the edge of the shirt. 

Fifthly, a gusset, a triangular piece of material, is usually added for reinforcement in the corner between the breast and the back. You will find this detail on all good shirts. However, there is only one manufacturer that uses this little detail as a marketing tool. The gussets on shirts from Thomas Pink are always pink. This is intended to remind the owner of the shirt of the maker every time he wears it, and would not work for Turnbull & Asser or T. M. Lewin. 

Sixthly, on good shirts the cloth of the sleeve is pleated several times where it meets the cuff. In addition there is often a small button above the cuff that prevents the sleeve opening up to reveal the arm in an unflattering way. It can also be unbuttoned to make rolling the sleeve up easier. On very good shirts this buttonhole is horizontal, and not vertical. On the very best shirts it is hand sewn, of course, just like all the other buttonholes. 

Seventhly, the more stitches a seam has, the more durable it will be, with about 20 stitches per inch (8 per cm) on a good shirt. Seams, even parallel double seams, are always sewn with a single needle. The advantages of this are that the seams are more precise, and the material in between them does not go wavy after washing. Material is used generously when good shirts are made, so that they do not slip out of the pants at the back or sides.