If you use analytics software like Google Analytics, you should check your stats for the past few days. If your traffic has dropped from what it was since last couple of days and has remained consistent, your website might have been penalized.
However, there could be various other reasons sudden traffic drops so make sure to analyze whether you still rank higher for your best keywords and haven’t lost traffic on them.
Also make sure to check that you are still getting traffic for those keywords for past few days or not. Another thing worth checking is to see whether you have lost traffic from referring sites like Facebook and Twitter too.
A thorough check of these things should give you a clear idea on whether or not you have been penalized by Google.