I think you need to rely on your memory. You cannot rely on the LastModified date since it gets changed just by clicking the Save button even it there´s no change.
This is what I do: I have a folder on the desktop where I keep all single files (classes, modules, forms) from the VBA project. After any change to a file I drag the file from the project explorer (ctrl+r) to the folder on the desktop. This is much more convenient than clicking File/File Export. Importing works the same way, just drag the file into the project explorer. (Note, if the file already exists, the imported one gets renamed.)
Saving all files this way instead of just backing up all the *otm file has another advantage: The project file can become corrupted, which would require to build it from scratch; that´s when you need the single code modules.