On the E-M5 you can tell if both the IBIS and OIS are engaged at the same time because they will "fight" each other and the stabilised live view preview will jerk a little as you move the camera around. I only have Panasonic OIS lenses that have a physical switch but when I use them on my newer E-P5 it will not allow the two IS systems to be active at the same time. As soon as the lens IS switch is set to on the IBIS deactivates. I would think (hope) that the E-P5 is also smart enough not to simultaneously run both IS systems with Panasonic lenses that don't have a physical IS switch
That's correct. The only way that you can enable OIS on a lens without a physical switch is to do so from the camera body. In the case of the E-P5, that means setting Lens I.S. Priority to 'On' and enabling IS in the Super Control Panel. If you have a lens with a physical switch, the setting there overrides the 'Lens I.S. Priority' setting. So if the OIS switch is on, you get OIS, otherwise you get IBIS or no IS depending on the camera's IS setting.