How to tell if Optical Image Stabilization is actually working on my iPhone 6 Plus

The optical image stabilization on the iPhone 6 Plus has to do with a tiny motor changing the lenses while the sensor is fixed to the body of the phone.

It only engages for still photos and due to the size of the motors and limited space inside the device, it will be almost impossible to hear the sound of it in operation.

The real test will be if your phone isn't capturing sharp still images that are well lit and have some contrast lines for the focus pixels to establish what the proper focus should be. I suppose you would need to mount an iPhone 6 and 6 Plus on the same physical mount and take the same picture (burst mode) or using something to trigger both shutters at the same time.

Also, you might try taking pictures from a moving car out the side window and see if you can detect any difference in the EXIF metadata between pictures taken while moving versus pictures taken when still. Apple encodes all sorts of details and I would start by looking at the Focus Mode field to see if OIS is indicated there.

That said, optical image stabilization on professional canon and nikon lenses costing $4000 and up isn't magic and just gets you one or two stops of exposure for a given shutter speed. The effects are not huge so you are gaining a slight boost in the amount of light that can be gathered and the shutter is allowed to stay "open" for longer than if the OIS were not helping to keep the image centered on the CCD.