Testing an argument for fallacies is said to focus on the concept of

Checking an argument for fallacies means examining its logic and reasoning.

Logos is the appeal that rests on logical structure and evidence.

A fallacy is a break in that structure, a flaw in the reasoning itself.

Credibility (ethos) and emotion (pathos) concern the arguer and the audience, not the validity of the inference.

Testing for fallacies therefore centres on logos.