Right Hand Grip
 

Both the left and right hand should be Vertical at Impact. The left and right hands should be 'parallel' (palms facing each other). A good check with the left hand is to make sure the score lines on the clubface 'matches' your flat left wrist at the top of the swing.
But how can you be sure that the right hand isn't turned under or rolled over too much? It can be hard to tell...but I stumbled across something recently...

If the orientation of the clubface matches your flat left wrist at impact fix and your right palm is facing your left palm when your grip is then taken, there should be no problem. Should there?

right hand grip check
 

So, if the left wrist is vertical, how does one ensure that the right wrist is also vertical (as opposed to turned or rolled) so that both palms are facing each other?

I found a good way to check is to go to impact fix (left wrist is flat, level and vertical; right wrist is bent, level, vertical), and then take the club to the top of the backstroke.
Monitor the amount of right wrist bend from impact fix address to the top.
If your right wrist bend has increased, this means your right hand is rolled over the grip too much.
If it’s decreased (flattened), your right hand is turned under too much.
If the amount of right wrist bend stays constant, then it’s perfect!
This gives a very desirable feeling of sameness throughout the stroke. Of course, the above assumes you have a flat left wrist all the way. I find this to be a useful check because it's not always obvious when your fingers are wrapped around the club.