You're right, sometimes Hermann's and some Greeks looks similar. Hermann's often get confused with Ibera Greeks (T. g. ibera). The easiest way to tell the difference is by looking at the plastron. Hermann's have (roughly) two black vertical bands whereas Greeks have larger splotches or even total black. Another, very obvious way to tell is the presence of a spur/nail/claw right on the end of the tail in the Hermann's. Greeks have no such spur. The shape of the head is also a big indicator. The pages below show good photos of the head and nose shape for each species.
Check out the eastern Hermann's photos on this page: