How to detect a Monad?

Many of us don't have a background on functional programming, and much less on category theory algebra. So let's suppose that we need and therefore create a generic type like

data MySomething t = ....... 

Then we continue programming, and using MySomething. What evidences should alert us that MySomething is a monad, and that we have to make it one by writing instance Monad MySomething ... and defining return and >>= for it?

Thanks.

Edit: See also this question: is chaining operations the only thing that the monad class solves?, and this answer a monad is an array of functions with helper operations