1. Sit in it. You have no idea how many people don't do this. Can you adjust it to an appropriate height so your knees are at a 90 degree angle and your feet are flat on the floor? When you sit comfortably, does your back feel supported? Remember, your chair should make it easy to sit with perfect posture.
2. Feel the fabric. Even check out the "ingredients" tag to find out the exact blend of materials. If you are buying an upholstered office chair, the fabric should be breathable and easily cleanable. Remember, you'll be sitting in this thing for hours on end, so pleather is never a good idea.
3. Take a spin. If you are buying a swivel chair, make sure it's loose enough that you can easily turn left to right, but not so loose that you are working hard to sit in one place. Some chairs allow you to adjust this feature, so if you're concerned, look for that too.
Before you buy any chair, be sure to ask about the return policy. Ask if you can return it if you don't like it — even if you have used it. It can be tough to know if the chair is the perfect fit until you have a chance to sit in it for a full day, so you want to be sure you can get your money back if it's a dud. Always keep those receipts!