Well, regardless of the GD&T standard utilized a fundamental rule of datums is that the "Datum feature must be readily discernible on the part".
You can read more about this in ASME Y14.5-2009, para. 4.8 (page 57).
Here's what I recommend:
- The engineering drawing should be clarified so that the datum feature is "readily discernible on the part". This can be done in variety of ways (part number on opposite surface, dimple, etc.).
- If this is a drawing must be inspected by you, I would contact the responsible engineering drawing owner and ask for clarification or alternative instruction. Hopefully the clarification is documented for future use.
- If you have no engineering drawing contact or support and you must inspect it I would inspect the part in two separate setups. What I mean by this is setup using one of the possible datum features then the other.
I recognize that this is redundant and extra work in addition to doubling the chances of rejecting the part due to an out-of-tolerance condition. But this will ensure that a functional part reaches assembly.