这是不可能的.***的办法是验证一个字符串是否可以有效地用作Base 64，尽管许多仅由ASCII文本组成的字符串也可以被解码为Base 64.

This isn't possible. The best you could do would be to verify that a string might be valid Base 64, although many strings consisting of only ASCII text can be decoded as if they were Base 64.