How do you detect if JavaScript is disabled?

Not with JavaScript — if it is disabled, none of your code runs. The detection has to live in the HTML itself.

<noscript>

The browser renders the contents of this tag only when scripting is off:

<noscript>
  <p class="warning">This site needs JavaScript to work properly.</p>
</noscript>

The CSS inversion trick

More useful in practice: assume JavaScript is off, then let a script prove otherwise.

<html class="no-js">
<head>
  <script>document.documentElement.classList.replace("no-js", "js");</script>
</head>

.no-js .requires-js { display: none; }
.js .js-fallback     { display: none; }

Because the inline script runs before the body paints, there is no flash of the wrong state. You can now style entire sections for each case rather than showing a single warning.

<noscript> can also carry CSS to hide interactive elements:

<noscript><style>.requires-js { display: none; }</style></noscript>

On the server, you cannot know

The request carries no signal about scripting. Anything server-rendered must assume nothing, which is why progressive enhancement is the real answer.

Build so the question matters less

Rather than detecting and apologising, make the core work without scripting:

• Use real <form> elements with action and method so submission works unaided, then enhance with fetch.
• Use <a href> for navigation, not click handlers on <div>.
• Render content on the server; let JavaScript add interactivity on top.

This matters beyond the rare user who disabled scripting: it also covers a failed script load, a network blip, an ad blocker, and the first moments before hydration — situations far more common than JavaScript actually being switched off.