is-weakset 
Is this value a JS WeakSet? This module works cross-realm/iframe, and despite ES6 @@toStringTag.
Example
var isWeakSet = require('is-weakset');
assert(!isWeakSet(function () {}));
assert(!isWeakSet(null));
assert(!isWeakSet(function* () { yield 42; return Infinity; });
assert(!isWeakSet(Symbol('foo')));
assert(!isWeakSet(1n));
assert(!isWeakSet(Object(1n)));
assert(!isWeakSet(new Set()));
assert(!isWeakSet(new WeakMap()));
assert(!isWeakSet(new Map()));
assert(isWeakSet(new WeakSet()));
class MyWeakSet extends WeakSet {}
assert(isWeakSet(new MyWeakSet()));
Tests
Simply clone the repo, npm install, and run npm test