They'll just do the usual retort that obfuscates the entire question by appealing to the abstract social nature of labor. If you're only at one spot on the assembly line and you contribute to a small part of the manufacturing of the commodity, it's a lot harder to measure how many commodities you make a day, since you never really make an entire commodity with your hands the way an artisan would. Someone milks the cow. Someone bottles the milk. Someone screws the cap on. Someone puts the label on. Someone makes the bottle. Obviously it's even more complex than that, but this is a simplified example. If a process of making a commodity is divided into 5 separate jobs, each worker only creates on average 20% of the commodity at a time. So if a given individual in that process does their job 100 times they really only "created" 20 commodities. And at this point the question becomes abstract enough for people to not really know exactly how much they're individually doing for the capitalist and it becomes harder for them to measure whether they're being paid the full value of their work or not. Intuitively of course most people know they aren't, because otherwise profit and hence surplus value/surplus product couldn't exist. And collectively the workers definitely are getting less than they give on average.