impdp exports the DDL of a dmp backup to a file if you use the SQLFILE parameter. For example, put this into a text file
impdp '/ as sysdba' dumpfile=<your .dmp file> logfile=import_log.txt sqlfile=ddl_dump.txt
Then check ddl_dump.txt for the tablespaces, users, and schemas in the backup.
According to the documentation, this does not actually modify the database:
The SQL is not actually executed, and the target system remains unchanged.