The flat IDE ribbon cables have 20 pins. They come in two types: 80 conductor and 40 conductor. The difference is pretty obvious when you look at them: The 40 conductor cable has a coarser appearance and looks similar to a FDD cable. The 80 conductor cable has much finer (thinner) wires. You will need the 80 conductor type to be able to achieve UDMA 4 which is what plextor recommends in their users manual (and also in page 2 of their data sheet - http://www.plextor.com/english/products/pdfs/PX716Ads.pdf )
Most mobos or systems shipped within the last 2 years that have IDE/ATA 66(DMA mode 4) or ATA 100 (DMA mode 5) hard drives use the 80 conductor type ribbon cables. Sometimes they will also ship a 40 conductor cable for opticle drives.
So: all you have to do is count the number of wires.