Try this to test if a file is a directory:
FOR /F "delims=" %I IN ('DIR /B /AD "filename" 2^>^&1 ^>NUL') DO IF "%I" == "File Not Found" ECHO Not a directory
This only will tell you whether a file is NOT a directory, which will also be true if the file doesn't exist, so be sure to check for that first if you need to. The carets (^) are used to escape the redirect symbols and the file listing output is redirected to NUL to prevent it from being displayed, while the DIR listing's error output is redirected to the output so you can test against DIR's message "File Not Found".