Identify the intended shape
Look at the jacket's construction before using the shoulder seam as a measuring point. On conventional tailoring, the seam and sleeve head can help you assess whether the jacket sits naturally. Raglan sleeves and oversized dropped shoulders deliberately place seams elsewhere.
Put on the layer you expect to wear beneath it. A jacket tried over a thin vest may feel very different over your usual shirt or knit. Fasten it as intended, then let your arms rest naturally instead of pulling the shoulders back into a formal pose.
Look from several angles
Check the front, back and side if mirrors allow. Notice whether the collar sits comfortably, the sleeves hang without an awkward twist and the upper back feels restricted. One wrinkle is not a complete diagnosis, but repeated pulling or an uncomfortable fit is worth taking seriously.
Move your arms forward as if reaching for a desk or holding a bag. A structured jacket will not move exactly like a sweatshirt, yet it should suit the activities for which you are buying it. Consider whether the fit works when seated as well as standing.
Keep alterations realistic
Shoulder alterations can involve more of the garment than shortening a simple hem. Ask a tailor to assess the construction before buying a jacket on the assumption that any problem can be corrected cheaply.
Try another cut if the shoulder area repeatedly feels wrong. A softer construction or a different sleeve shape may suit your needs without requiring reconstruction. The goal is a jacket you can wear naturally through an ordinary day, not one that only looks convincing when you remain still and arrange your arms for the mirror.
