Good fit does not mean tight fit. It means the garment and the body appear to agree. The shoulder sits where it should, the cloth falls cleanly and the silhouette has enough ease to move without losing its intention.
This is why an understated piece with the right proportion often looks more luxurious than a louder piece with the wrong one.
Start at the shoulder
For jackets and structured shirts, the shoulder is the least forgiving point. The seam should meet the natural end of the shoulder without collapsing inward or extending into an unintended shelf.
From there, look at the chest and back. Fabric should skim rather than pull, and movement should not create strain lines that remain when you stand naturally.
Read the full silhouette
Do not judge one measurement in isolation. Jacket length changes the apparent position of the waist; trouser rise changes the apparent length of the leg. The eye reads the composition before it reads the details.
A small alteration can transform that composition. The best tailoring does not announce the work—it simply makes the wearer look resolved.