Telling if a jacket is a Blazer/Sport Coat or if it's a Suit Jacket: If the wool is woven in a subtle weave such as birdseye, nailhead or sharkskin, it is a Suit jacket. If the wool is flat and slick and when bent and held to a light it nearly reflects, it is a Suit jacket. Pinstripes. If it has a pattern like POW, pencil thin large checks, windowpane, or a windowpane over a POW, however its subtle, meant for subtle differentiation and not statement making, it's a Suit Jacket. Lastly, if the jacket has a 3 button front closure and 4 button cuffs and may possess one of the previous qualities but your not quite sure, 99.9% of the time its a suit jacket. It's a Blazer if the cuffs have 3 buttons or less, it has extremely bold patterns, doesn't have a pants waist size on the manufacturer tag, as a fabric finish like hopsack or twill, has patch pockets (98% of the time), is made of cashmere blend or all cashmere (95% of the time), or has a fabric composition such as linen, silk, wool, it's a blazer. Also metal buttons are almost always a sign its a blazer. If it's unstructured its a blazer, if its self lined, it's a blazer. Overall Sport Coats are more substantial and less delicate if that makes sense...the drape differs, the fabric choice and cuts differ in thickness and finish. Also you'll often see a V shape formed by the 2 halves of the jacket under the bottom button when laid on its back, a sign it was Cut for a suit. Blazers, on their back laid flat, both sides will stay fairly true and straight....I think that pretty much does it.. ADVICE: When in question or doubt, put it back, it's a suit jacket.