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A new black hole search method has just yielded fruit, and boy is it juicy. Astronomers have found a stellar-mass black hole clocking in at around 70 times the mass of the Sun - but according to current models of stellar evolution, its size is impossible, at least in the Milky Way.

https://www.sciencealert.com/an-impossible-black-hole-has-been-found-in-the-milky-way-galaxy

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One possibility, however, could be a fallback supernova, in which material ejected from the dying star falls immediately back into it, resulting in the direct formation of a black hole. This is theoretically possible under certain conditions, but no direct evidence for it currently exists.

"Impossible" only in the headlines. 

BHs can acquire mass by means other than 'evolution'.
We recently witnessed the merger of two stellar sized BHs to yield a larger one.
And there are examples of stellar sized BHs swallowing adjacent ( orbiting ) stars.

How would you tell the difference after the fact ?