How to check if an object is a Promise?

Whether it's an ES6 Promise or a Bluebird Promise, Q Promise, etc.

How do I test to see if a given object is a Promise?

How a promise library decides

If it has a .then function - that's the only standard promise libraries use.

The Promises/A+ specification has a notion called thenable which is basically "an object with a then method". Promises will and should assimilate anything with a then method. All of the promise implementation you've mentioned do this.

If we look at the specification:

2.3.3.3 if then is a function, call it with x as this, first argument resolvePromise, and second argument rejectPromise

It also explains the rationale for this design decision:

This treatment of thenables allows promise implementations to interoperate, as long as they expose a Promises/A+-compliant then method. It also allows Promises/A+ implementations to “assimilate” nonconformant implementations with reasonable then methods.

How you should decide

You shouldn't - instead call Promise.resolve(x) (Q(x) in Q) that will always convert any value or external thenable into a trusted promise. It is safer and easier than performing these checks yourself.

really need to be sure?

You can always run it through the test suite :D