How to find the schema name of a data pump dmp file?
Use the sqlfile= parameter of impdp to generate a file containing all of the DDL/DML in the dump. 

For example:

[oracle@oel61 ~]$ impdp phil/phil directory=oracledmp dumpfile=phil.dmp logfile=phil.log sqlfile=philddl.txt

Import: Release 11.2.0.2.0 - Production on Wed Mar 13 15:15:03 2013

Copyright (c) 1982, 2009, Oracle and/or its affiliates.  All rights reserved.

Connected to: Oracle Database 11g Enterprise Edition Release 11.2.0.2.0 - 64bit Production
With the Partitioning, Automatic Storage Management, OLAP, Data Mining
and Real Application Testing options
Master table "PHIL"."SYS_SQL_FILE_FULL_01" successfully loaded/unloaded
Starting "PHIL"."SYS_SQL_FILE_FULL_01":  phil/******** directory=oracledmp dumpfile=phil.dmp logfile=phil.log sqlfile=philddl.txt 
Processing object type SCHEMA_EXPORT/USER
Processing object type SCHEMA_EXPORT/SYSTEM_GRANT
Processing object type SCHEMA_EXPORT/ROLE_GRANT
Processing object type SCHEMA_EXPORT/DEFAULT_ROLE
Processing object type SCHEMA_EXPORT/TABLESPACE_QUOTA
Processing object type SCHEMA_EXPORT/PRE_SCHEMA/PROCACT_SCHEMA
Processing object type SCHEMA_EXPORT/SEQUENCE/SEQUENCE
Processing object type SCHEMA_EXPORT/TABLE/TABLE
Processing object type SCHEMA_EXPORT/TABLE/STATISTICS/TABLE_STATISTICS
Job "PHIL"."SYS_SQL_FILE_FULL_01" successfully completed at 15:15:05

[oracle@oel61 ~]$ 


Looking for CREATE USER DDL statements in the file should show you what schemas are needed in order to perform an import.

For example, from my test dump:

-- new object type path: SCHEMA_EXPORT/USER
-- CONNECT SYSTEM
 CREATE USER "PHIL" IDENTIFIED BY VALUES 'S:924B2E756404611021428644B4DF06A4A7BAB886837FCCFA510151E0FC44;181446AE258EE2F6'
      DEFAULT TABLESPACE "PHILDATA"
      TEMPORARY TABLESPACE "TEMP";
-- new object type path: SCHEMA_EXPORT/SYSTEM_GRANT
GRANT UNLIMITED TABLESPACE TO "PHIL";
GRANT CREATE SESSION TO "PHIL";
-- new object type path: SCHEMA_EXPORT/ROLE_GRANT
 GRANT "DBA" TO "PHIL";
-- new object type path: SCHEMA_EXPORT/DEFAULT_ROLE
 ALTER USER "PHIL" DEFAULT ROLE ALL;


The only drawback is that if the .dmp file is large the resulting SQL dump will be huge.