What JayWill said is true 99% of the time... If you have the SuperUser app there your normally good to go.. Occasionally something goes wrong in the rooting process and Superuser doesn't stick even with the app listed.
Only way to tell is download and open up an app requiring root access like ES File Explorer or Titanium Backup and when you open the app it will prompt you to accept root access or not... That will tell you right there if your okay. If there is something wrong with your root it will prompt that root access isn't available. Then you just have to run your rooting process over again.