Android - How can I tell if I have root?

Use terminal emulator (a popular free/open-source terminal emulator is Android Terminal Emulator, available from F-droid or Play Market), open the emulator and type

su

you will know with the message that appears after if phone is rooted or not.

Edit:

There is an app just for this now : Root Checker

Connect your phone in developer mode via USB and start an ADB shell

adb shell

If you see a # at the beginning of the line your phone is rooted, if it's a $ try to execute the su command, and if you get a # now, your phone is rooted, but if you get an error that su can't be found it's probably not, or the su binary is simply missing.

There are also apps on the market that test if your phone is rooted.

There is an application available in the market called Root Checker, that can verify if your phone is rooted, and if you have BusyBox installed.