How to find if SQL server backup is encrypted with TDE without restoring the backup
I up-voted Brent's answer, as his scenario could definitely muddy the water on whether the backup contained TDE data.

However, if you've had TDE enabled for a while, it seems that RESTORE FILELISTONLY (Transact-SQL) might provide the information you're after.  There is a column on the result set called TDEThumbprint which "Shows the thumbprint of the Database Encryption Key. The encryptor thumbprint is a SHA-1 hash of the certificate with which the key is encrypted."

I looked at some of my backups which were both TDE encrypted and not TDE encrypted.

The backups of my TDE databases had the certificate thumbprint in that column and the backups that did not have TDE databases had null.