How can you tell if you have scoliosis?

See your Dr. See an orthopaedic specialist to really determine if you have scoliosis, you will have a physical exam followed with x-rays.
Get a friend to look. Bend over like you were going to touch your toes and have someone else look at the surface of your back (mid back and low back) as though it were the horizon. If one side (right versus left) is higher than the other, consider seeing a scoliosis specialist. Standing up straight, scoliosis can cause one shoulder to look higher than the other or make one shoulder blade stand out.
Potentially. It can be surprisingly hard to tell if you have scoliosis without an X-ray, partly because there are often curves above and below the main curve that compensate for the main curve and bring the trunk back into an upright position. Signs include shoulder elevation, obliquity of the pelvis, and the presence of a "hump" in the back on forward bending. Don't hesitate to be seen for a diagnosis.