Well, apart of obvious encryptions like bas64, which are indetified instantly, there are encryptions which you cannot tell they are present. aes256, etc.
This is becouse those algorythms generate a string which looks random, but it is not. If we have something like
asfsds4agr4ยท$56ds4$a:24ds64r75r
How do you tell how is it encrypted? It could be anything. Read this (second message)
http://forums.devshed.com/security-and-cryptography-17/identifying-the-encryption-type-373304.html