Access to a directory

Hi guys,
How I can check if I have access to the installation folder just after "!insertmacro MUI_PAGE_DIRECTORY"? I don't know the installator tell me that I haven't access to the folder after finishes it, I want just when it prompts me the destination folder to tell me if I haven't access to there.

Very thanks!

If you type an invalid (not accessible) directory, next button is disabled.