The RhadaTip /rt hc frame can be used as a gem optimizer.

This method isn't very handy when the item has multiple sockets, or when you want to check your gear gems (assuming that those items are already gemmed, you will need a non-gemmed tooltip like the one from AtlasLoot in order to use this method).

RhadaTip has a gem optimizer.

This is what it does:

Running RhadaTip gem optimizer to find out the best gemming combination for my currently equipped pair of leggings.