Look at the link or the sender: Say it is a gift card from Target. The link address
would not be anything like Targetgifts@hotmail.com It would only come from Target.com Same with anyplace you do regular business. The last part of the email will be their web address.
If it is someplace where you do regular business, and they want you to update security information or passwords, NEVER use the link. Open your internet browser and go to the site like you normally would. If they want security updates it will tell you after you log in. If not, you know the email was fake.