If we know that $\dot{q} = \frac{\partial H}{\partial p}$ and $\dot{p} = -\frac{\partial H}{\partial q}$

And $Q$ and $P$  are $q$ and $p$ at a later time step. How could we prove that:

$Q = q + {\Delta}t\frac{\partial H}{\partial p}(q,p) $,
$P = p - {\Delta}t\frac{\partial H}{\partial q}(q,p) $

Is not symplectic,

 While:

$Q = q - {\Delta}t\frac{\partial H}{\partial p}(q,p) $,
$P = p + {\Delta}t\frac{\partial H}{\partial q}(q,p) $ 

is symplectic