There are a number of readily adoptable metrics you can utilize to keep a close eye on your affiliate program. It's a good idea to use a blend of these to give a more rounded picture.
But it's important to note that effective collaboration is about more than this. It's about communicating with your affiliate. It's great to stay on top of their performance. It's even better to work with them on making their operation more productive. It's to everyone's benefit, after all.