We taught Uni the difference between left and right hand. it's super easy and is very impressive. Since she already knows "Paw" all you have to do is have her do "paw" and you come in with the treat on one side only. for example to teach right hand/paw come in towards her with the treat on her right side. Say "right hand" since the treat is closer to her right hand she will naturally use her right hand to get it, instead of the left. Make sure it is HER right and left and not yours. My bf kept confusing himself which was right and left! LOL Do it enough and you can do it later with just the voice command and you won't even need to do the hand signal.
Here's a vid of Uni when she was about 4 months.