It’s nearly impossible to buy a suit off the rack and have it actually fit you. Suits must be tailored to fit your body in the sleeves, arms, legs, tail, and waist.
The only place a suit can’t be modified is the shoulders. If it doesn’t fit in the shoulders, that means it doesn’t fit, period. There’s no tailoring magic that will make it so — just don’t buy it, no matter how good a deal it is.
But what does it mean if the shoulders don’t fit? Basically, a suit jacket’s shoulder should not extend farther than your natural shoulder.
You can try this test to be sure.
Lean your arm against a wall while wearing your suit jacket. Your natural shoulder should hit the wall before the shoulder of the jacket does.
If the jacket’s shoulder hits first, the jacket does not fit you. And as we all know, fit is the most important thing to consider when purchasing a suit. You’ll risk looking like you got caught wearing your father’s suit.