It is incredible the difference a good pair of padded cycle shorts make. After years of trying different shorts, this is the list of things I look for hopefully, this will help you decide.
Chamois pad
Chamois is a French word, named after the soft leather made from a goat-like animal. The first chamois was made from their leather, hence the name. Nowadays, they are mostly made of synthetic materials. Look for a well-made pad with no hard ridges.
Legs
Leg length varies. The one thing is to for is that they have a grip that will make sure that it stays in place. Look for wide grip, I would stay away from the pure elastic as they tend to cut in. You don't want sausage legs.
Waist
This can be a tricky one. It wants to be strong enough to stay on, but you don't want to feel you are being cut in half. Look for a wide waistband, preferably higher at the back you don't want any unsightly gap.
Stitching
Look for a flatlock stitch, even so, watch where the contacts the body. There is a difference between proper flatlock stitching and faux one. Flatlock stitching requires specialist equipment and is a lot stronger. Look for good quality stitching.
Wicking (or how well moisture gets away from your skin)
How the materials used to deal with moisture. Lets facing you are going to sweat and get rain on and you don't want any wetness sitting next to the skin.
A chamois without glue helps.
The Material feel
Not one on most lists, but I think it is essential. I buy clothes on the way that they feel as well as the practical and look. I can't stand short that are rough to touch.
Final thoughts. Sit on a bike saddle, preferable your bike saddle to see how they feel. 'You pays your money and takes your choice' The less expensive the cycling shorts generally the rougher the material, pad and stitching and of course they won't last as long. Buy the pair that suits your body and go out and enjoy your ride!
Photo credits: Main photo, short by Rob Percy, Chamois courtesy of Drpad.