Using the example below one can view the contents a compress tar file without un-compressing the file.
# if the file is compressed using compress
$ uncompress -c file.Z | tar -tvf –
-rw-r–r– 2054 202 0 May 20 22:17:19 2009 a
-rw-r–r– 2054 202 0 May 20 22:17:19 2009 b

$ uncompress < file.tar.Z | tar -tvf –
-rw-r–r– 2054 202 0 May 20 22:17:19 2009 a
-rw-r–r– 2054 202 0 May 20 22:17:19 2009 b

# if the file is compressed using gzip
$ cat file.tar.gz | gunzip | tar -tvf –
-rw-r–r– 2054 202 0 May 20 22:17:19 2009 a
-rw-r–r– 2054 202 0 May 20 22:17:19 2009 b

• Unix