There isn't really a feature that displays your study streak, but if you go to the "Stats" panel on your dashboard, you can choose to display an activity heatmap that shows you an approximation of your study levels each day. As long as the box isn't white, you'll know that you studied that day!
Also, if you have your Kao garden enabled, you'll see that some of the permanent quests involve study streaks for specific things like vocabulary, kanji, grammar, and sentences.
There is no way to see your study streak aside from the ones @Inkheart mentioned. But on the bright side, there is no one shouting at you at 11.50 in the evening to do at least one thing a day to keep the streak going (still terrified. *hang on, what's the green thing*)
Also small trivia, if you use the web browser, the shown www. link next to something .me has a number and it is your number as the # person that joined renshuu.
