Shoulders and collar: buy for these two
A blazer is bought at the shoulders and collar, because those are the two places tailoring cannot economically reach. The shoulder seam must end where your shoulder does — an overhang collapses into a divot on the upper sleeve, and a short seam strangles the arm. Check it side-on in the mirror; the sleeve should fall in one clean line from the seam.
The jacket collar should lie flat against the neck with no gap behind it and no bunching below. A gap means the jacket is cut wrong for your posture; a roll of fabric means the back needs work. If shoulders and collar sit right, the jacket is a candidate. If either fails, walk away at any price.
The jacket body: closure, waist, length
Fasten the jacket's single button, or the top of a double-breasted front. The fabric should close without a strain line pulling across the bust and without gaping at the buttonhole. Tighter, and the front bows open under any movement; looser, and the jacket reads boxy and borrowed.
The waist should nip in slightly, giving the jacket shape without pinching when you sit. Length is checked two ways: the hem should just cover the curve of the hip, and your fingertips should roughly reach the hem with arms relaxed. A cropped cut is a deliberate style choice, not a fit failure, as long as it is chosen on purpose rather than landing there by accident.
Sleeves and the sliver of shirt
Jacket sleeves end where the wrist bends, letting a shirt or blouse cuff show a centimetre — the small detail that separates tailored from approximate. No cuff showing reads as outgrown; several centimetres showing reads as borrowed.
This assumes the shirt underneath fits first — sleeve to the wrist bone, collar closing with two fingers of room — which is why how a shirt should fit comes before blazer shopping, not after. Sleeve shortening is the cheapest, most routine alteration there is, so never reject an otherwise correct jacket over sleeve length. Buy the shoulders; pay the tailor a few hundred rupees for the wrists.
Trousers: rise, seat, break
Suit trousers sit at the natural waist — the narrow point near the navel — not down on the hips where casual trousers live. Worn low, the rise collapses and the leg line breaks; worn where they are cut to sit, the leg falls in one clean line and the jacket meets them properly.
The seat should follow you without hammocking below or pulling tight across. From the knee down, the trouser should hang straight into one shallow break — a single soft fold where fabric meets shoe. No break is sharp and current with a tapered leg; a full puddle is the most common failure in every formal outfit. Hemming is a hundred-rupee fix; there is no excuse.
What a tailor can and cannot fix
Buy the closest fit at the shoulders, then budget a visit to the tailor — nearly every good suit you have seen was altered. Routine and cheap: sleeve length, trouser hem and break, waist in or out a few centimetres, darts through the jacket body for shape at the bust and waist. Possible but costlier: reshaping the hip, shortening a jacket slightly, adjusting the collar.
Effectively impossible: shoulders, the armhole, and lengthening anything beyond the seam allowance. That is the entire buying strategy — nail the unfixable, negotiate the fixable. For the big occasions this suit will serve, from job interviews to formal dinners, the two hundred rupees of hemming matters more than the two thousand you might have spent on a better label.