How does one determine which claim of sexual assault should be taken seriously, and which one should be ignored?

Some accusations of sexual assault that involve political figures seem to be taken very seriously and are investigated publicly and thoroughly. Those made against Donald Trump, and Justice Kavanaugh come to mind as examples.

Others do not.
Those made against Joe Biden is an example. The only person talking in that case is the woman.

How does an obselrver figure out what is the governing principle that identifies which accusation should be investigated and which one can be ignored.