If the plaintext input bytes are limited to the printable ASCII range (0x20 to 0x7E inclusive) then the following statements will be true of the encoded output:
+will only appear if the input bytes contain either~or>/will only appear if the input bytes contain a?
...and in both cases, this will only happen if the particular input byte occurs at a position 3n+2 bytes into the input. The corresponding output byte will appear 4n+3 bytes into the output.
This means if the input doesn't contain any of ~, >, ? then the output will be purely alpha-numeric (trailing = padding notwithstanding).
