As far as what to look for,it would depend on whether it's an early one or late one. Early one was most likely a 2 speed hydrapower,late might be a 3 speed Over/Under. I'd just check that to make sure it's shifting. Either way,it will go in to free wheeling in under drive. They tend to hang up in gear and shift out hard in direct or overdrive,that's normal,just the way the unit's made. When that happens,you just pull them in to underdrive.
The dual speed PTO will have a shift rod sticking out a little bit in the far lower right hand corner of the housing. There's a set screw with a locknut going up from the bottom to hold it in place.
Just normal wear and tear is about all there is to look for. That one looks pretty sharp in the picture.