For data checks you could use the following rsync option in regular intervals, e.g. once a week or month:
-c, --checksum This changes the way rsync checks if the files have been changed and are in need of a transfer. Without this option, rsync uses a "quick check" that (by default) checks if each file’s size and time of last modification match between the sender and receiver. This option changes this to compare a 128-bit checksum for each file that has a matching size. Generating the checksums means that both sides will expend a lot of disk I/O reading all the data in the files in the transfer (and this is prior to any reading that will be done to transfer changed files), so this can slow things down significantly. [...]
You probably could also use this together with the “dry run” option to verify files between the Syncthing end points, but would essentially need to dis-allow any activity on/usage of the source PCs during the check, to avoid “false alarms”.