If you training main skills just see how much DMI do you get. If it suddenly dropped by ~3x of DMI gained, then it is capped. But still that cap is just a soft cap, it means it trains only 3x slower and it is possible to squeeze out few more pops.
I had squeezed out ~12 and 15 pops for perennial allstars, ages 23-31
. But at some point, it got so ridiculous that DMI gained training main skill and DMI lost due to stamina drop was negative or around 0, like +100 for a 1+m DMI guy. They reached hardcap. Along the way it might be just my own observations, but it seems they got another 3x slow down hit after quite alot of pops and later after few more another 3x. I am pretty much convinced it goes in big steps.
Usually MVP's do not cap bellow 100%, but sometimes they do, sometimes it doesnt show on calculations if that player has high sublevels of some skills. Average i would say is around 102-103% for MVP. But seen as low as 94. Same goes to HOF, rarely you see 94-97 but sometimes they go beyond ATG not capped.