If you are familiar with Unix command line utilities; I would suggest using Cygwin.
You can use one out of many "diff utilities" to show you the difference between the contents of two single files, or all files in two different folder. During Cygwin's installation process, the package you need to install for this to work is called
diffutils. If you don't change any of the installation options, they should be installed as part of the default installation.
This is likely the fastest way to do what you want, especially if the folders you are comparing contain a lot of files. You can find
diff's complete manual here, or by typing
man diff in the Cygwin terminal window. There are a lot of different options you can specify to change the utility's behaviour.
If you need to compare files on two different computers, you can share the folders and diff them over the network, or copy the files to a temporary storage device, such as an USB stick or a portable harddrive (depending on how big the files are, of course).
You can find the difference between two folders like this.
Folder1, which contains these three files:
Then I created
Folder2, which contains these three files:
Now, I simply diff the two folders against each other, like so:
diff Folder1 Folder2
This produces this output:
Only in Folder1: TextFile1.txt
Only in Folder1: TextFile2.txt
Only in Folder1: TextFile3.txt
Only in Folder2: TextFile4.txt
Only in Folder2: TextFile5.txt
Only in Folder2: TextFile6.txt