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