With lots of changes to SO, some questions and answers became obsolete. The letter box is gone. So how to see if the daily cap is hit?
What happens when the daily cap is hit? Is the overflown reputation lost?
Click on your rep (the number itself) next to your name, gives a nice view of your rep. Next click on "Graph". You'll see a horizontal line at 200. You hit the rep cap when you've got green verticals touching the horizontal rule or going over it.
Check at /reputation page, and yes, you start losing those reps, when you start seeing numbers inside square brackets -