Menswear
Shoulders
The shoulder seam should end where your shoulder ends. If it hangs past the bone or the fabric dimples, the jacket is too big. Shoulders are hard to alter, so check them first.
Chest and buttoning
Button the jacket. It should lie flat without the fabric pulling into an X. If it pulls, try the next size up.
Sleeves
The jacket sleeve should end near your wrist bone, with about half an inch of shirt cuff showing.
Length
A common guide is that the jacket should cover your seat. Too short can look boxy, too long can swallow you.
Trousers
Trousers should sit comfortably at your natural waist and break lightly over your shoe, with no bunching at the ankle.
Bring the right shirt and shoes
Try a suit with the shirt and shoes you plan to wear. It changes how everything sits.
Not sure what you are seeing in the mirror? Come in and ask.