When you stream Xtrabackup to agzipped tar file, it also contains the information about the binary log position where the server was at when the backup was started. This information is needed to start the lave at the same position again
time nice -n 15 xtrabackup --backup --slave-info --safe-slave-backup --stream=tar | gzip -c > /var/backup/percona.dump.tar.gz
In order to get that position without extracting the archive file, you can use the following command
tar -xOf /var/backup/percona.dump.tar.gz xtrabackup_binlog_info
You can list all files in the archive like this:
tar -ztvf percona.dump.tar.gz
Note that both operations can take quite some time and resources.