For Symphony Mk II modules, not all Mk II modules are marked with a "MK II" sticker on the front of the module to indicate this is a MK II module.
However this does not necessarily mean that the module is an older original Mk I module.
You can determine if a Symphony I/O module is MK II or MK I based on the shape of the PCBA board.
For original MK I modules all PCBA boards will have a partial cutout as shown below:
For current MK II modules, all PCBA boards will be fully rectangular in shape as shown below: