Hey guys! This is asked sooo much in chat, so I thought I might aswell cover it on the forums.
When you want to know someones real name all you have to do is:
/realname *That players nickname*
My nickname is Crafty, so to find out what my realname is, you would do:
You don't have to worry about colors, just type in the letters/numbers that the nickname consists of.
I hope this has helped <3