You have no chance of doing this task without knowing how the interface works. The interface basically determines HOW you are going to receive or transmit data. Where does your data come from? is it on chip? is it off chip?
The data will arrived clocked 64 bits per clock (or per valid cycle- depending on how the interface works). If the length is always the same then you know how many words you will receive. If its variable then there will be some way to determine how long it is - likely a packet header or some sideband signalling.
THis is why its important to know how your interfaces work. Without knowledge of the interfaces, this task impossible.