Read the Product Reviews. Read the advertisements and read the manufacturer's promotional material. Order the instruction manuals from the manufacturers. The Sourcebook offers detailed QST Product Reviews dating back to the '60s. Look around the bands for people using the rig you are considering.
Service can be important, too.
Ask about the service offered by local dealers and equipment manufacturers. I think I know which radio I want. There is no substitute for hands-on experience with the radio of your dreams. Find a local ham or dealer who has one. With luck, you'll be able to secure an invitation to visit and operate it for a while.