I can see in "Users" tab you have groups for
week, month, quarter, year, all, but no
day. Is there an easy way to see which users have hit the rep cap today?
Even considering the answer by Nick, this would be a somewhat difficult thing to ever actually deliver because, one could argue, that what Nick says isn't particularly true.
You're asking to know who has "rep capped"... and that is not the same as knowing who has earned at least 200 reputation in a given day. We must remember that getting accepted answers, bounties and the association bonus all can cause daily rep to be much more than the reputation cap and, as it is possible for someone to win a Quidditch match without catching the Golden Snitch, so too is it possible for someone to earn thousands of rep in a day and still not rep cap - it's not particularly likely, but it is possible.
So, to answer your very specific question, no, there is no way to see who has hit the rep cap on any given day, Monday or otherwise.