Well, the good (or at least better) OpEd authors will provide references. Or at least, they'll provide enough detail on their source material that an interested reader can track down the reference - which is all that's really required if the touchstone is that there really is a valid reference for the facts being provided or asserted.

An example of this would be economist Thomas Sowell.

Anyone who outright fails to provide a means of verifying their claims, regardless of the field, must be taken as suspect. Except maybe if they're commenting on something already so well known that no new references are required...but situations like that are easy enough to spot.