How to predict (know) the products of the reaction SnO+NaOH?
I've discovered the answer, at least I guess...

The given reaction is 

$$
\ce{SnO + 2NaOH -&gt; Sn^{2+} + O^{2-} + 2Na+ + 2OH-}
$$

The hidroxy breaks in $\ce{H2O + O^{2-}}$. This released oxigen will get together with the oxigen from the $\ce{SnO}$, to form $\ce{O2^{2-}}$. 

$$
\ce{SnO + 2NaOH -&gt; Sn^{2+} + \underbrace{O2^{-2}}_{\textrm{nox = -4}} + 2Na+ + H2O}
$$

The oxidation state of this molecule is -4. Performing the double replacement reaction we get

$$
\ce{SnO + 2NaOH -&gt; SnO2^{-2} + 2Na+ + H2O}
$$ 

And finally,

$$
\ce{SnO + 2NaOH -&gt; Na2SnO2 + H2O}
$$

What is already balanced.