If you have the key, trying to decrypt it is THE solution ;-)
Otherwise a compression test may be a quick (but bad) solution depending on the data being transmitted.
(quite efficient if you transmit text, or other redundant informations)
If you can't save space by compressing it :
It's probably that it's
- Already compressed
- Encrypted
"
Only
Bad
Coders
Badly
Code
In
Perl" (OBC2IP)