Measure the dimensions of the corner that you plan to place the cabinet. Bring these measurements with you when you shop.
Check online and read consumer magazines for feedback on various corner cabinets. Narrow your search based on the experience of others.
Choose a style and finish that will complement your existing décor.
Find a cabinet with the proper dimensions. If it isn't going to fit, there's no reason to waste time looking at it.
Check the construction of the cabinet. The pieces should fit well together and have a solid feel.
Look for adjustable shelves to allow more storage options.
See if you can select which side the door mounts to the cabinet frame. This is a nice option if you have other furniture near the corner and don't want to bang the door into something.
Find out what kind of mounting hardware you will need (if any) to install the cabinet. Some cabinets include the hardware; some do not.
Bring the cabinet home and install. Save all your materials and receipts in case you have to return it.