I have started checking hems first when looking at anything, because more information is concentrated there than anywhere else on the outside of a garment.
Depth
How much fabric is turned up.
A deep hem — an inch or more — adds weight at the bottom, which helps the garment hang properly, and leaves fabric available for lengthening.
A narrow hem uses less fabric, costs less, and provides neither benefit.
On trousers, on skirts and on coats, hem depth affects how the garment moves, and it is the sort of thing you notice as an impression rather than as a detail.
Method
A blind hem uses stitches that catch only a thread or two of the outer fabric, so nothing shows from the front.
A topstitched hem shows a visible line of stitching, which is appropriate on casual garments and reads as cheap on tailored ones.
A hand-finished hem is essentially invisible and appears on a narrow band of garments.
A fused hem, glued rather than stitched, exists and fails.
The curved hem problem
Where skill becomes visible.
Turning up a straight edge is trivial. Turning up a curved edge means the folded fabric has more length than the surface it is being attached to.
Which must be eased in, distributed evenly, or the hem puckers.
Look at the hem of a shirt or a curved jacket from a few feet away. Ripples mean it was rushed.
Vents and slits
Where the hem meets a vertical opening, the corner is a construction test.
A properly mitred corner produces a clean diagonal fold with no bulk.
A folded and stitched corner produces a lump.
Coats and skirts show this clearly and it takes a second to check.
Weighting
Some tailored garments have chain weight or lead tape sewn into the hem to improve hang.
Which is a genuine construction technique with an obvious cost and no visible benefit, so it appears only where somebody was thinking about how the garment moves rather than how it looks on a hanger.
Trouser hems specifically
The break — how much the fabric folds where it meets the shoe — is a proportion decision that has swung considerably over time.
What matters mechanically is that the hem is level at the front and slightly longer at the back on most cuts, since the leg is not vertical.
A hem that is level all the way round will look short at the back.
Turn-ups add weight and hold the leg line, and they cost additional fabric length, which is why they were traditionally associated with more expensive trousers.
What can be altered
Shortening is straightforward as long as the hem shape allows, which is why tapered trousers are harder to shorten a long way — the leg opening changes as you go up.
Lengthening is possible only if there is fabric in the hem, which is why hem depth matters at purchase.
Faded crease lines from the original hem sometimes remain visible after lengthening, particularly on dyed cotton, and cannot always be removed.
The habit
Checking the hem takes three seconds and covers fabric quality, construction care, and future alterability.
It is the closest thing to a single reliable test I have found, and it is on the outside where you can look at it without turning anything inside out in a shop.
Sleeve hems specifically
A separate case with its own considerations.
Shirt cuffs should sit at the wrist bone, which is where the hand begins to widen, and this is checkable in a mirror in five seconds.
Jacket sleeves should show a small amount of shirt cuff, which requires the shirt sleeve to be correct first — a point people get backwards.
Functioning cuff buttonholes on a jacket look considerable and make sleeve alteration far more expensive, since the length must be adjusted from the shoulder instead.
Which is worth knowing before buying, because it converts a routine alteration into a substantial one.
Skirt and dress hems
Where the shape of the hem determines what is possible.
A straight hem shortens easily. A circle or bias-cut hem does not, because the circumference changes as you go up and the fabric hangs differently.
Bias-cut garments also drop after hanging, since the fabric elongates under its own weight along the bias.
Which is why a bias garment should hang for a day or two before being hemmed, and why a professional will insist on it.
The rolled hem
On lightweight fabric, a narrow rolled hem is the only appropriate finish, and doing it well by machine takes practice.
Hand-rolled hems on silk scarves are the visible marker of quality in that category, producing a slightly raised edge rather than a flat stitched one.
Coat hems
Where the weight of the fabric makes construction most visible.
A heavy coat hem needs interfacing to hold its shape, or it will ripple and hang unevenly within a season.
The lining should sit slightly shorter than the outer hem, with a fold of ease at the bottom so it does not pull the outer fabric up when you move.
A lining stitched tight to the hem is the reason some coats gather oddly at the front.