Some indications can be taken from Some Heuristics for Evaluating the Soundness of the Academic Mainstream in Unfamiliar Fields:

Low Hanging Fruit Heuristic

if a research area has reached a dead end and further progress is impossible except perhaps if some extraordinary path-breaking genius shows the way, or in an area that has never even had a viable and sound approach to begin with, it’s unrealistic to expect that members of the academic establishment will openly admit this situation and decide it’s time for a career change.

Thus: You are likely having imaginary expertise if all low hanging fruits in your field have been taken.

Ideological/venal Interest Heuristic

You should be doubtful about your expertise when

things under discussion are ideologically charged or a matter in which powerful interest groups have a stake.

Note that these concern your expertise despite you actually being an expert.