It seems like $P\neq EXP$ is much easier than $P \neq NP$. How can I prove $P \neq EXP$?
(Well, after all I want to know any proof technique of proving there does not exist any algorithm of certain time complexity to prove a decision problem.)
It seems like we have to construct a problem where there's no way but to check all the cases which is exponential time...
I guess this is what you are looking for.