How to check if a variable is an ES6 class declaration?

If you want to ensure that the value is not only a function, but really a constructor function for a class, you can convert the function to a string and inspect its representation. The spec dictates the string representation of a class constructor.

function isClass(v) {
  return typeof v === 'function' && /^\s*class\s+/.test(v.toString());
}

Another solution would be to try to call the value as a normal function. Class constructors are not callable as normal functions, but error messages probably vary between browsers:

function isClass(v) {
  if (typeof v !== 'function') {
    return false;
  }
  try {
    v();
    return false;
  } catch(error) {
    if (/^Class constructor/.test(error.message)) {
      return true;
    }
    return false;
  }
}

The disadvantage is that invoking the function can have all kinds of unknown side effects…