The most reliable source for the software is the forums or archived driver sites.

Look for the largest chip. If you see an ATMEGA or PIC16F877A , you likely need the "Willem USB v0.x" software. If you only see logic gates (74HC244, 74HC373) and a DB25 parallel connector, you need the classic "Willem EPROM v0.97g" or "v0.98d" software.