As a continue to this question: [How can I know if a pile in "Grundy's game" have a winning strategy?](https://math.stackexchange.com/questions/4172845/how-can-i-know-if-a-pile-in-grundys-game-have-a-winning-strategy):

I'm talking about ["Grundy's game"](https://en.wikipedia.org/wiki/Grundy%27s_game).

I understand from the answer that if I have one pile of $20$ is loosing position because $n(20)=0$,  

but if I split it to $19,1$ , I get: $n(19)+n(1) = 3>0$ - and this winning
position.

So my question is: how I can know if I have winning strategy or not?
Assume that is my turn now and I have one pile of $20$, I can win or not?  

If yes, what is me next move?

Thank you!