You can go to Flightradar24, for example, and search on a route or flight and get a history of what aircraft was used and what is scheduled. Often the MAX9 is scheduled for the longer routes, but it sometimes gets swapped out for the MAX8. This can give you a sense of the probability if the MAX9 will be used, but no guarantee.

The last Copa flights I took were CNF-PTY-IAD. While PTY-IAD was a shorter flight, there were less swaps, possibly because it was flying to/from the US capital.