How can employer confirm my certificate

Short Question: How can an employer confirm the authenticity of our certificates?

Long Question: Though each certificate has a serial number, it is easy to print out fake ones. How can potential employers confirm the authenticity of the certificates we get from the courses?

Unless required by law employers usually don’t verify the documents provided by applicants. If you don’t have the skills you claimed to have you simply get fired. If you provided fake documents you might get sued and/or reported/prosecuted.