My "posture check" is to take my stance and think "Coudl I play any other sport standing like this?"
The answer is always "NO" if I am in a bad posture.
i.e. not an atheltic stance

I find its easier than trying to figure out if I have too much knee flex or if my back is straight.
For some strange reason you dont think about any of this when you try hit a baseball or throw a frisbee... go figure.