Decode a string with unknown encode method

Here is an interesting (maybe), stupid (maybe) question. I got a string below:

w0nm%2BtHxOs8z%2FTyiLxWvNc8Rb%2B0Cnr%2FO5iHLktTyQ%2FmOq2YPNtDUJ8R%2BuEPwbajXq6rc2iWU0kKWFFquxKj%2BGw%3D%3D

I tried to convert HTML characters, and the string becomes:

w0nm+tHxOs8z/TyiLxWvNc8Rb+0Cnr/O5iHLktTyQ/mOq2YPNtDUJ8R+uEPwbajXq6rc2iWU0kKWFFquxKj+Gw==

still no clue what's about. From the pattern, it looks like base64 encoded. But a开发者_运维百科fter decode, the result is again meaningless:

I:3</5o!Cf6'~Cm%BZ

1. How do I know what's the content without knowing its encode method?
2. Which are the common decoding methods?