There are a few different ways you can test an email design:
Test in different email clients: One of the most important things to do when testing an email design is to view the email in a variety of different email clients to ensure that it is displayed correctly. Some popular email clients to test in include Outlook, Gmail, Yahoo, and Apple Mail;
Use email testing tools: There are several tools available that allow you to test your email design in a variety of different email clients. Some popular options include Litmus and Email on Acid. These tools typically allow you to view a preview of your email in different email clients and devices, as well as check for any technical issues such as broken links or formatting issues;
Test on different devices: It's also a good idea to test your email design on different devices, such as desktop computers, laptops, tablets, and smartphones, to ensure that it is responsive and displays correctly on different screen sizes;
Send test emails: Another option is to send test emails to your own email address or to a few other people to see how the email looks in their email clients. This can be a good way to catch any issues that might not be apparent when viewing a preview of the email.
Overall, it's important to thoroughly test your email design to ensure that it is displayed correctly and looks professional in a variety of different email clients and devices.