Any good ways to combine only the necessary css and js for a web application

I improved the interface structure of the web application and did things such as combining and minimizing css / js files, version files, and creating some templates. However, I looked at some web tools, and he says there are a ton of unused js / css. Are there any suggestions for -detection of the completely necessary code -combination, minimization and correctness of the correct code for the page I'm basically looking, as a logical suggestion, any code samples would be great. I specifically use python, cheetah patterns, yui-compressor, modified time for versions, mod_rewrite and gzip on apache