I have a YM1110D with the powershift transmission. Here is how I checked to see if my internal clutches were slipping. Since the regular clutch also engages the pto, I just put the tractor and pto in gear and began driving. Then I pushed the brakes hard enough to stop the tractor from moving, and then checked to see if the pto was turning. It was turning, while the tractor was not moving. That meant that the internal clutches were slipping, not the regular one. But in doing this, the engine lugged down alot, and began to smoke. So I figured that was a heavier load than I was ever going to put on the drivetrain.
I think you are on the right track. If the internal clutches are slipping, then either the clutches are worn out (I haven't heard of anyone replacing them yet) or they are not getting enough hydraulic pressure. The most common problem is leaking o-rings in the hydraulic shifting mechanism (which is really a fancy hyraulic valve) not allowing enough pressure to the clutches. Hopefully, someone else with more experience can provide some better information.