In the universe of Star Trek, Vulcans and Romulans were one species as late as the fourth century CE, about two thousand years prior to the TNG era.
Is there a reliable or standard way to tell the difference between a Vulcan and a Romulan in the absence of any other context (such as genealogies, cultural leanings, etc.)? E.g. will a standard life energy scan correctly differentiate the two? Is there a medical exam that will do this? Is a genetic test required? Will a genetic test reliably differentiate the two, or are there frequent "outliers" that don't match the expected racial genotype?
I recall that there was one episode of TNG where a member of Starfleet was caught lying on their induction paperwork (they were actually 1/4 Romulan rather than 1/4 Vulcan as claimed), but this was only after some time had passed and they had been serving for some time, so presumably nobody noticed anything peculiarly "Romulanesque" about the person during the standard Starfleet medical exams.
As to why there might or might not be a way - consider that real-world humans have been evolving for a lot longer than 2000 years. We don't have a reliable test that will, say, out someone as specifically Slavic, say, as opposed to Greek, or Etruscan. We rely on trends for which there are always major exceptions and complications.