I think that for the current round of turns, the question of what you gained or trashed should be honestly answered. There's nothing in the rules that say you cannot quickly buy a mountebank, put it on your discard pile, and clean up your play area. Doing so may hide your true intentions, and if people aren't paying close attention, they'll miss it. For the sake of full disclosure, I think it's reasonable to ask what people gained/trashed on their last turn.
Beyond that turn, of course, it's based on memory. I wouldn't expect someone to say, "How many Provinces have you bought so far," so that you can figure out the Province allocation among all players. If you aren't keeping count, then you lose out later (unless you're on Isotropic of course). I lost a game this weekend because I missed a vital piece. I knew I had a Duchy and a Province. I watched the purchases of the others. One player had a Province, one player had a Duchy, and one player had a Duchy and an Estate…or so I thought. When I purposely ended the game to secure my victory, I saw that the third player actually had a Province. I did not see him buy it. He wasn't trying to hide that fact. It was early enough that I wasn't 100% focusing on the victory cards. Of course, my other mistake was not counting the Province pile. I would have known that I was off when I saw nine Provinces instead of 10. Ah well.
But, if I had noticed that round that I didn't catch what he bought, I would have asked. And if he was the type to smarmily say, "No," then I wouldn't play with him again. Games are held in far too casual of an atmosphere to be smarmy about it.