File Copying across Network

I have two apps on different PCs on a network ( peer to peer at the
moment ). App_A copies a file to a folder on PC_B where App_B reads and
processes the data.

The problem is that App_B seems to detect the presence of a 0 byte file
before the data is transferred!

Is there an elegant way for App_B to detect that the entire file has been
copied? I solved this type of problem once before by looking at the file
size but in this case the file is compressed and has unknown size.

TIA

Arthur Richards