Your chances of it working are pretty high since you took it within 72 hours. I know it sucks, but you're going to have to wait until your period is due and test if you don't experience any sort of bleeding by that time.
Don't freak yourself out with deciphering symptoms or anything like that. Plan B has a lot of junky side effects that are pretty similar to a horrible round of PMS or your typical early pregnancy symptoms-- so really, any weird feelings, irregular bleeding or discharge, moodiness, etc... It’s all fair game and could very well just be the Plan B.
The good news is a simple home pregnancy test after a missed period will be accurate.