I did brew cask install cmake, but when I was about to brew install something, brew wants to install cmake. How can I let brew know that there's already cmake available?
You can’t. Homebrew and Homebrew Cask are separate projects that install things in different ways. Homebrew assumes you don’t have cmake if it’s not installed through Homebrew so it’ll refuse to install even if a cmake binary exists in the PATH. It has no way to ensure this cmake binary is indeed the cmake it’s looking for.
A hacky way would be to edit the formula you want to install (brew edit <formula>); remove the line with depends_on "cmake" => :build; install it (brew install <formula>); then add the line back again.