1. Check your shoulder pads
Shoulder pads should sit squarely on your shoulders, and the fabric of the sleeve skimming the widest point of your shoulder curvature.
If you see the fabric sink inwards or underneath the pad, the jacket is too big.
Conversely, if you are able see the curve of your shoulder, then you may need to go one size up.
2. Buttons should not be pulling
A sure sign that your blazer is too small is when you see the fabric around your buttons pulling backwards. In cases like this, letting out the waistline of the blazer might help.
If you are able to fit two palms between the blazer’s buttons and your body, then the blazer is too big.
3. Watch the collar
A perfectly fitted collar should frame the base of your neck beautifully. Some tailors give you a bit of space, but it should never exceed one finger’s space.
4. Your sleeve should end squarely on your wrist
Menswear tailoring sometimes allows for blazer sleeves to end an inch higher than their wrist to show off a shirt cuff or a watch, but typically, womenswear tailoring doesn’t always accommodate that.
To check that the sleeve fits, bend your wrist and make of note of that point as the spot where a sleeve should end.
From there, you can decide if you want to shorten it by up to an inch to show off your watch.
5. Your hem should remain flat
Much like how a well-fitted blazer should not show any signs of pulling or rumpling, the most tell tale way is to check the lining.
Feel around the jacket as you’re wearing it, and check that the lining is not bunching up underneath the jacket.
If you find that there’s a feeling that the lining is stuck somewhere (and no amounts of tugging can straighten it), it’s probably too small.
Also, watch the hem to make sure it remains flat. If you see that it starts curving in towards you, it means that the blazer is too small, and the lining is being pulled and stretched (hence lifting the hem inwards).