You are testing the product.
Use an accessibility toolbar for a first test. For example, the WAVE toolbar is available for Firefox and Chrome. The automated tests are not perfect but they give you a first impression of accessibility.
Help for tools:
Take a look at the structure
Assess whether the general structure makes sense to you. Look at whether the headlines and the clickable elements are clearly identifiable. The headings bookmarklet also allows you to check whether the headings structure has the correct structure.
Navigate by keyboard
Operate all functions using the keyboard You should have no problem navigating to all elements and implementing all actions.
Desktop: Put the mouse away.
Phone: Connect an external keyboard to your phone.
Use a screen reader
Operate all functions with a screen reader. On the desktop you can also use NVDA free with Firefox or a demo of JAWS with Internet Explorer. These are the two most common combinations. VoiceOver is already pre-installed on iOS devices.
Help for a screen reader setup:
Testing with NVDA:
Testing with JAWS:
Testing with VoiceOver: