[cuspycode]

But how do you even distinguish appearance of thought from actual thought, objectively? Of course you can always argue that actual thought is something that is subjectively experienced, but that leads to solipsism, doesn't it?

[Der_Einzige]

Solipsism is what you get from accepting the Cogito "I know I exist for sure, and only my own existence can be verified by the cogito"

To answer your question, I claim that you can't. Radical skepticism ("we live in the universe of the evil demon"), which is what I advocate for, means that you can't even claim that you exist. See the works of Max Stirner for further elaboration of the implications of this for Philosophy.