It's not just twin engine boats that have a reverse (RH, right hand) rotation motor, many single engine ski boats use RH motors. The new Correct Craft Ski Nautiques have RH 351 Ford based power, even the older ones from the early 70s with Chrysler Marine 318s were RH.
The most common Chrysler Marine had to be the 318LA. I've seen more of those although the 440 and 426 aren't too far behind. The 360 came to use as Chrysler Marine market share started to drop but they are out there as they were the mainstay after the 440 was dropped. The 340s aren't that common but I have seen a few. 273s and 318A polys are also found in the early to mid 60s boats. The Chrysler flathead six was very common to the old 40s-50s wood boats.
If the motor is apart, easiest way was posted above, check the crank for rear main reverse knurling. If the motor is still together with the trans, look for a tag on the motor or trans that will tell if it is RH or LH.
Here's what Chrysler Marine offered in 1967, before the 340 was introduced.
