Each addon (mod) typically has a .txt file in the folder that is the Install instructions.

Be careful though, as some authors do not break down the codes they are wanting you to add to various files - they just tell you to upload this file and that file.

So, if you know that certain files have been altered either by previous addons or by customizing your store - you will have to do compare/contrast of code to see what actually is being added. Winmerge is helpful in that task.