Always try on both shoes. No one has perfectly matched feet. When shopping for shoes, wear hosiery and carry along an extra pair of socks. That way you can try on the shoes with the appropriate hosiery to make sure the shoes are not too loose or too tight. Shop for shoes in the afternoon to get the best fit.
Fluid retention causes feet to be wider at the end of the day.
Check the length. There should be about a half-inch of space between your longest toe and the end of the shoe. Feel the lining all the way to the toes. It should be completely smooth and free of lumps and bumps. Look for shoes with round or square toes.