Ensuring a website is fully responsive across different screen sizes is essential for user experience and SEO. Here’s how you can test and optimize website responsiveness efficiently.

1. Use Browser DevTools for Quick Testing

Most modern browsers include responsive design mode. In Chrome, open DevTools (`Ctrl + Shift + I` / `Cmd + Option + I`), then enable Device Mode (`Ctrl + Shift + M`).

2. Test on Real Devices

Simulators can be helpful, but nothing beats testing on actual devices. Use mobile phones, tablets, and desktops with different screen resolutions.

3. Use Online Responsive Testing Tools

Platforms like BrowserStack, Responsinator, and wp engine Google Mobile-Friendly Test allow testing across multiple devices and screen sizes.

4. Check for Breakpoints & Fluid Grids

Use CSS media queries to ensure proper layout adaptation:

@media (max-width: 768px) {
          .container {
            width: 100%;
          }
        }

5. Optimize Images for Different Viewports

Use responsive images with the `srcset` attribute:

<img src="default.jpg" 
             srcset="small.jpg 480w, medium.jpg 1024w, large.jpg 1600w"
             sizes="(max-width: 600px) 480px, 1024px" 
             alt="Responsive Image">

6. Test Performance & Loading Speed

A responsive website should also be fast. Use Google Lighthouse to check performance and identify improvement areas.

7. Verify Touchscreen & Navigation Usability

Ensure interactive elements (buttons, links) are touch-friendly by keeping tap targets at least 48px in size.