You can check J Crew's swimwear size chart for a start. They have measurements there. It depends on your size. Really, the easiest way to find out is to just look at yourself, or have someone else look at you.
If you:
-have trouble wearing waist-cinching styles like belts and wraps
-don't have a very defined waistline
-have a long inseam for your height
-find many tops to be too long
-don't have a lot of space between the bottom of your ribcage and your hipbone
you're probably have a short torso
If you:
-like waist-accentuating details like belts
-have a defined, gently curving waist
-find many tops too short
-wear a short inseam for your height
-have lots of space between your ribcage and hip bone
you probably have a long torso
BUT...but but but...
Not everyone has a long or short torso. Many people are well-balanced and fall somewhere between the two or have a combination of long and short torso issues (for lack of a better word).