is-async-function 
Is this a native async function?
Example
var isAsyncFunction = require('is-async-function');
assert(!isAsyncFunction(function () {}));
assert(!isAsyncFunction(null));
assert(!isAsyncFunction(function* () { yield 42; return Infinity; }));
assert(isAsyncFunction(async function () {}));
Tests
Simply clone the repo, npm install, and run npm test