Most poker rooms have an option where you can tick a box to muck losing hands. Some internet poker sites have it displayed as muck losing/uncalled hands (or even muck losing uncalled hands).
Once this boxed is ticked it is not possible to show your cards to your opponents other than in the event of a showdown.
I am often asked whether it is better to have the muck losing hand button ticked or un-ticked. Having the box ticked speeds up the game considerably as losing or uncalled hands are automatically folded, but showing your hand may enable you to gain some advantages in subsequent rounds. For example, to get action in subsequent rounds you might choose to show your opponent that you were bluffing.
I firmly believe that in limit ring games it is better to have the box ticked to speed up the game. This is because bluffing is not so much of an issue in this form of poker and having the box ticked will increase the number of hands per hour. For heads-up, no-limit or pot-limit poker, I think there are definite advantages to showing your hands from time to time, to generate more action in subsequent rounds or to show your opponent that you are strong to prevent them from bluffing. I am of the view that the advantage gained outweighs the extra time takes in this the aforementioned styles of poker.