How do you check whether the password is strong or not

Question

I want to ensure that passwords meet strong security standards, but I’m unsure of the best way to verify password strength.

• What characteristics make a password strong? (length, complexity, uniqueness?)
• Are there tools or algorithms that can automatically score password strength?
• How can I check password security without storing it in plaintext?

Looking for best practices and reliable methods to test password strength securely.