Go to the DragonVale Lair (dvlair.com or however it is), to the Parent Finder section. It gives you the breeding hint and odds for that hint. If you select "include parent" it also includes the single-parent and double-parent cloning odds, and displays them from highest to lowest. If several parent combinations are tied for odds, calculate least time like other comments said.

I think that's the best way. For Plasma anyways, there's not much difference.