Only a person in the hand can request to see hands at showdown.

However, If the loosing player quickly mucks his hand at showdown, no one can see his hand b/c the Dealer cannot take cards out of the muck.

Smart players quickly muck as soon as they see they are beat so no one is able to see their hands - not even the dealer can pull cards out of the muck.

So, if you want to see a hand.... force the other player to open first... or tell the dealer before opening that you want to see all hands showndown.

Many players make the mistake of pompously displaying their hand without realizing the value in seeing opponents hands and how they played it vs you.