Given a linux username and a password how can I test if it is a valid account?

Question

So my question is straight forward given a linux username and a password how can I test if it is a valid account?

Answer 1

If you are concerned about security (which you should be), the accepted answer represents a security risk by leaving the plaintext password in the ~/.bash_history file. With this in mind, it would be better to try logging in, or perhaps removing this entry from the ~/.bash_history.