You have clues from the previous waves. If you look at the A and B waves just prior to this last C wave they are both clearly 3's. That means this has to be some sort of flat and not a zig-zag. A zig-zag is what I have drawn on my euro daily chart. Anyway a C wave is always comprised of 5 smaller waves meaning a flat is always 3,3,5 and a zigzag is always 5,3,5. Also and ending diagonal is almost always 5 waves but can be 9 (rarely). This ending diagonal is only going to be five because the throw over has already occurred and it only breaks out of the trend line on the final push. This is actually an expanded flat because the C wave is quite a bit larger than the A and B wave. It's really not as difficult as it sounds. If you are interested in learning Elliott Wave theory I recommend the book Frost and Prechter wrote in the 70's, Elliott Wave Theory. I also have some good ebooks I will send you for free if you would like. They aren't as in depth as the hardback, but they are free.