On the assumption you are represented by a credible and experienced EB-5 Attorney, you and your Attorney should each receive a receipt notice from the USCIS. Your Attorney should give you a copy of the cover letter and the I-526 petition, and if applicable a copy of the mail return receipt from the post office or a private courier service. You may personally call the general inquiry section of the USCIS and ask if they have in their records the date of filing the I-526 petition.