When I started in this hobby almost 40 years ago that was said about pretty much all reef fish species, with the few exceptions of of those species that have a clear sexual dimorphism (pipefish, seahorses, mandarins, Anthias,...).
Today it can be done for quite a number of fish.
There is definitely at least one way to sex any fish - the one scientists use. The only problem with that method is that the fish isn't alive by the time you know its sex.
Tangs are capable of changing sex according to what I could find in scientific research papers. But I didn't find anything about the trigger mechanism.
The most common trigger among reef fish is the position in the group hirachy. In general the dominant one on the top becomes the male (anemonefish are the exception with the females being the larger and dominant ones in a pair). This could definitely also be how it works for the more social tangs.
Other triggers could be age or size.
In essence, if you really want a pair give it a shot and add another one.
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