Often I have been asked to provide the ticket number as well as the boarding pass (or, in the olden days, a copy of the ticket receipt as well as the boarding pass).

I am not sure if a boarding pass alone would be accepted, as it could be difficult to mine the necessary information about booking class, etc. without a ticket number. Not impossible, mind you, but not trivial either.

If the OP purchased the ticket using a credit card, the ticket number will always appear on the statement with the purchase. If he/she used a travel agent, they should have a record of the ticket number, as I would expect they keep invoices for some time. If the ticket was bought with a stack of cash at the ticket counter... it could be more difficult to track down.