Automated checkers: Browser extensions like axe DevTools are excellent for catching about 30-50% of common issues, such as most color contrast failures or missing form labels. The axe DevTools extension is built on axe-core and rarely has false positives. Do not rely on WAVE. It often has false positives.
Manual checks: Your keyboard is your most powerful testing tool. Navigate through your page using only the keyboard (tab, shift+tab, enter, space, arrow keys). Can you see where you are and operate every control? The ANDI accessibility tool provides valuable information as you test manually.
Assistive technology: The best way to understand the user experience is to test with a screen reader. NVDA (free for Windows) and VoiceOver (built-in on macOS/iOS) are the most common.
Color contrast: The WebAIM Contrast Checker can be used from a website and does not require an installation on your system. It is also free to use.