But beyond that I would dig deep into using the product. If I was going to assess the engagement (DAU/MAU) of a social network I would look at how people are commenting. Or maybe send 20 friend requests/messages to random people and see how fast on average they respond. You can get a feel for certain things just by using the product.
Otherwise I would just reach out and start a discussion. Some partners will be shy to give up metrics, but you usually can pull it out of them over time.
But that key predictor is different for every business. Finding it can be tough. But once you find it you increase partnership success chances by a lot.