How do you know you have a full packet

[george r smith]

All,   If I have learned anything from socket code it is that you can never be sure if you get a partial or a full packet. The question is if a packet is encrypted (des3) how do you know you have received the full packet ?   Obviously you can not check for a string or a delimiter that would tell you that the packet is complete.   What do you do in this case.   thanks grs