Since we cannot read people’s minds, we prove intent in the legal process through circumstantial evidence, or through direct evidence of intent such as communications or writings.

The TSA need not publicize their wins to prove effectiveness. Guilt can only be established in court; deterrence (which is what we’re talking about here) requires a much lower standard.

Your argument can also apply to the converse position, as well. Given the absence of evidence of success, can we reasonably accept that such successes do in fact exist? It'd be well within the TSA's interests to publicizing their successes, both for the agency's own operations and as a deterrent against would-be terrorists. They wouldn't even have to necessarily get into specifics.

Consider the TSA's bad press. There are years of stories detailing pervasive fail rates in detecting smuggled weapons or explosives by inspectors, misconduct and other scandals, and a number of viral videos showing incidents with groping and other bad behavior. If the TSA had major successes under their belt, it'd offer some rather useful political capital to deploy during the game of hardball that is the federal budget process. And the screeners' union, the AFGE, would be positively thrilled to be able to leverage those successes in support of extending full Title 5 protections and collective bargaining rights, not to mention General Schedule pay, to TSA screeners. Pushing back against the idea that screeners are incompetent would be of immense benefit to both the agency and the union.

Beyond that, a failed plot on US soil would lead to criminal charges against the conspirators. There should be evidence of major TSA successes in court records, and yet, nothing.