How I can know the sign in password?

If you forgot the Password that you created, then you have 3 options to reset it.

If you forgot your Apple ID password - Apple Support

Since you viewed the Related Article on the top of this page about Stolen Device Protection, you will see additional steps that may be required, such as using Face ID and performing the steps at a familiar location or waiting for the security delay.

If you have forgotten your password, you can reset it through your email

loodolif wrote:

If you have forgotten your password, you can reset it through your email

It's actually a little more complicated than that. See the link Mac Jim ID provided above.