How does the IRS know if I'm recently married?

It is always better to report your correct status.

They don't know. But, if you file as Married Filing Jointly you and your "spouse" must both include your SS numbers and sign the form under penalty of law. Filing as married if you are not is called "fraud."

They don't know unless you tell them.. there are certain tax advantages for filing married and advisng them of your marrital status will work for you.