Base64 DecodeString believes "password" is encoded

When attempting to determine is a string is encoded by using base64.StdEncoding.DecodeString() the string “password” is considered to be encoded. Thus when attempting to decode the string, malformed ciphertext error is returned.

An example can be seen here: Go Playground - The Go Programming Language

The expected result would be a error.

Should this be considered a bug or an anomaly?

Is there a better, more reliable method to determine is a string is encoded?

Given the bytes 165 171 44 194 138 221, password is a valid (the only valid) base64 encoded representation of exactly those bytes.