There should be a "Practiced Today" checkbox
Sometimes I don't remember if i've practiced or not, and it's super disappointing for that to be the reason a streak ends.
You can see this is you go to your home page, and click weekly progress, if the bar for the current day isn't filled at all, you haven't done any practice, else, if it has something filled, you have practiced today :)
I lost my 50 plus day streak because I was using my iphone app. And, I couldn't tell whether I'd studied that day or not on my iphone. (I thought I'd already studied.) So, it would be nice if Duolingo had some way to indicate this on the apps.