Is there any way to track when a TrueCrypt volume is encrypted or decrypted?

I don’t believe you can unencrypt a TrueCrypt volume. You can remove files from the volume, which decrypts them, but your only options with the volume (aka Container) is to continue using it or delete it. Whether there are files inside or not is up to you.

What might work better for you is to use a folder locking program. Here’s a link to the best one I’ve found Folder Guard

I’ve tried others but they all sucked.