If you do rclone size -vv then you’ll see an error for every undecryptable file name
$ rclone -vv size secret:
2018/12/12 21:49:18 DEBUG : rclone: Version "v1.45-DEV" starting with parameters ["rclone" "-vv" "size" "secret:"]
2018/12/12 21:49:18 DEBUG : Using config file from "/home/ncw/.rclone.conf"
2018/12/12 21:49:18 DEBUG : file.txt: Skipping undecryptable file name: illegal base32 data at input byte 4
Total objects: 0
Total size: 0 Bytes (0 Bytes)
2018/12/12 21:49:18 DEBUG : 2 go routines active
2018/12/12 21:49:18 DEBUG : rclone: Version "v1.45-DEV" finishing with parameters ["rclone" "-vv" "size" "secret:"]
$
You can at least identify the files like that.