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