is there a way to detect screens in a pdf

Adobe Acrobat Pro's Prepress tool is the solution for your problem. I assume you are looking for a color that uses more than one of your CMYK and spot colors (e.g. 40% black plus at least one other color). You have to create a new preflight check:

Now you have to create a check for the number of components or the number of CMYK components:

This will only work if your file is using only CMYK/Spot colors. If you need this to work with other color spaces, you need to do a lot more to find such files.

If you want to use some low level automation, you can e.g. create a preflight droplet, and then just drop a number of files on that droplet (I would not do all 5000 at the same time, you will have to experiment how many you can safely process in one batch), and preflight will then sort them into two directories. For a more advanced automation, you may want to look into the server solutions offered by Enfocus or Callas Software.

I am no longer working in a print based environment, so I have not followed all the print based enhancements to the Preflight tool, so that's all I can contribute to this discussion.

[I moved your question from The Lounge to a more relevant forum: Editing & Exporting PDFs]

What do you mean by "screens", exactly? And what do you want to do with this elements once they are detected?