We can validate a base64 encoded string using a nice regex (regular expression). The following regex can be used with any programming language. I would use php for the example. The regex is as follows.

^[a-zA-Z0-9/+]*={0,2}$

Which will also detect the usage of = or == at the end of the string (and only end).

A function geared specifically toward this:

<?phpfunction is_base64_encoded()
 {
     if (preg_match('%^[a-zA-Z0-9/+]*={0,2}$%', $data)) {
           return TRUE;
      } 
     else {
             return FALSE;
       }
 };
 is_base64_encoded("iash21iawhdj98UH3"); // true
 is_base64_encoded("#iu3498r"); // false
 is_base64_encoded("asiudfh9w=8uihf"); // false
 is_base64_encoded("a398UIhnj43f/1!+sadfh3w84hduihhjw=="); // false
?>