My landlord came to me and said she did not receive my rent check.?

How do I know she or her assistant didn't loose my check between the drop box and their office. She want's a late fee because it was not received by the 6th of the month. Due to certain circumstances I have had to pay a late fee in the past, however I am in the right this time as the monies are in my account. How do I know someone on her end did not loose the check? She has a box on the back of our community mailbox for rent payments. How do I know she didn't loose my check? Thanks for answer