Ask any collegue who worked with you in that timeframe. Ask them to prepare a letter, how they know you in company, what timeframe, your skills and duties (they shoud match specified in LCA). They should also provide their contact number if INS wants to contact them.
Ask them notorize it (most banks do it free) and you can submit a copy of that leter for RFE. If INS trusts, they may approve. If not they can verify it.
In any case, you and your collegue should be able to prove, if INS asks for more proof. You have to prepare for that.
INS might ask you or your collegue to submit evidence that he worked in that company in that timeframe with you. Or they can directly call the company to verify.