Not sure who you’d ask at the credit union, but would think it would be fairly easy to inquire there. If it was your father’s account, he might have to be the one to contact them. You could also check with the title company or attorney who handled your closing(s) as they may have a copy of the settlement statement and cashier’s check that would show where the down payment funds came from.
Proof of ownership should also be indicated by the recorded deeds on the property. They will indicate the date of transfer and the legal owners of the property. It should also disclose the purchase price. If there is not all the information that you need, I would recommend contacting the title company that handled the closing to see if they have the records.