Hermetic File Monitor is an interesting tool to monitor the files you indicate, and so verify that they have suffered no changes. The slightest variation detected in any file in a folder will be communicated to you.
Hermetic File Monitor lets you process and monitor unlimited folders at the same time; you can even configure it so that it also monitors the subfolders inside.
Of course, Hermetic File Monitor lets you filter the type of files being watched, and you can specify, for example, that it only monitors XLS files, or at specified regular time intervals. The results obtained are saved in reports.