Most fit problems can be fixed by a tailor. One cannot: the shoulders. That is why we start there.
1. Shoulders
The seam should end where your shoulder ends. If the sleeve head collapses or juts past your arm, try the next size. No alteration puts this right.
2. Collar
The jacket collar should sit against your shirt collar all the way round, with about a centimetre of shirt showing above it. A gap at the back of the neck means the balance is wrong for your posture.
3. Chest and waist
Button the jacket. The lapels should lie flat and the cloth should not pull into an X around the button. You should be able to slide a flat hand inside.
If the shoulders fit and the button closes without strain, the rest is a tailor's afternoon.
4. Length
A classic jacket covers your seat. With your arms at your sides, the hem should sit around the middle of your hand.
5. Sleeve
About a centimetre of shirt cuff should show. Sleeves are the easiest thing to alter, and the first alteration is free with every Harrow & Finch jacket.