Microsoft Password check is a free online tool to help you determine if your password is really strong or not ?

A strong password should appear to be a random string of characters to an attacker.

Each character that you add to your password increases the protection that it provides many times over. Your passwords should be 8 or more characters in length; 14 characters or longer is ideal.

It should include a combination of uppercase and lowercase letters, numbers, and symbols. Link.