Can I detect if my script is being processed by Webpack?

You could also do this -

typeof __webpack_require__ === 'function'

I'm guessing this might change at anytime so use with caution. :/

in Node.js global.global is cycle reference, Webpack is not creating this cycle:

function is_node() {
    return typeof global !== 'undefined' && global.global === global;
}

Put this in your webpack config under plugins:

new webpack.DefinePlugin({
    'process.env': {
        NODE_ENV: JSON.stringify('production'),
        APP_ENV: JSON.stringify('browser')
    }
}),

With that you can check if you're running in a browser or not this way:

if (process.env.APP_ENV === 'browser') {
    const doSomething = require('./browser-only-js');
    doSomething();
} else {
    const somethingServer = require('./server-only-js');
    somethingServer();
}

if (process.env.APP_ENV !== 'browser') {
    const somethingServer = require('./server-only-js');
    somethingServer();
}

Because these environment variables get replaced during the build, Webpack will not include resources that are server-only. You should always do these kinds of things in an easy way, with a simple, direct compare. Uglify will remove all dead code.

Since you used a function before and the function is not evaluated during build, Webpack wasn't able to know what requires it could skip.

(The NODE_ENV-variable should always be set to production in production mode, since many libraries including React use it for optimisations.)