is-symbol 
Is this an ES6 Symbol value?
Example
var isSymbol = require('is-symbol');
assert(!isSymbol(function () {}));
assert(!isSymbol(null));
assert(!isSymbol(function* () { yield 42; return Infinity; });
assert(isSymbol(Symbol.iterator));
assert(isSymbol(Symbol('foo')));
assert(isSymbol(Symbol.for('foo')));
assert(isSymbol(Object(Symbol('foo'))));
Tests
Simply clone the repo, npm install, and run npm test