Tool

Simulate mobile devices with Device Mode (Chrome DevTools)

Google Chrome Developers · Chrome for Developers Free Beginner

This is the free, built-in way to see exactly where your landing page breaks: open DevTools, toggle the device toolbar, and drag the viewport from a narrow phone width up to desktop. It is the fastest feedback loop because it is already in the browser you use every day, no signup or install. Treat it as a starting point for spotting the break, then confirm on a real phone since the simulator cannot reproduce everything.

• Toggle the device toolbar (Cmd/Ctrl+Shift+M) and drag the edges to watch the layout at every width, so you catch the exact point it breaks.
• The Show media queries view highlights which CSS breakpoints are firing, which is usually where a desktop-only layout falls apart on mobile.
• It simulates from your laptop, so do a final check on an actual phone before you ship.