As the title says: how do I know if I've reached my daily rep cap? Is there a simple yes/no indicator at least? Even better, is there a place where I can see "this is how many rep points left that you can claim today that is subject to the cap"?
You can click the envelope next to your name in the top of this page, click Today and if you have less than 200 reputation points earned, you still haven't reached the cap.
Actually I just found out that the Downvoter's method is not 100% reliable. I appreciate his effort in creating the picture, so I made my own (note: question titles are greyed out in this picture).
As you can see, according to the summary, I gained 193 points of reputation yesterday. This could mean that I may be able to get 7 more points. However, if you look at the itemized list, you see that I did not get any points for the last two items, since I've already reached my cap.
So I think this is the definitive way of telling if you've reached your cap or not: when you go to your Reputation/Today tab and see items you no longer get points for. In this case, even if the tab title says that you have < 200 points, you're guaranteed to have already reached your cap.
Note that this is a sufficient but not necessary condition, but that's fine if you want to be safe.