You can simply use the following syntax:
[root@blub ~]# tar tzvf mytar.tar.gz
That will show you the list of files in the tar. t means test
[root@blub ~]# tar tzvf mytar.tar.gz myfile.txt
-rw-rw-r-- scm/scm 373093723 2010-09-17 17:58:21 myfile.txt
You can do the same with a target file and test that one specific file really belong to a tar.gz
Finally you can extract that single file.
[root@blub ~]# tar xzvf mytar.tar.gz myfile.txt