What is the Arduino Uno default sketch?
Some may have the "Blink" sketch pre-installed, but many don't. However, when there is no sketch installed the board sits in bootloader mode waiting for you to upload a sketch.  While it's in bootloader mode the LED pin pin D13 blinks.

If your LED is blinking about once a second then it's the Blink sketch.  If it's rapidly blinking three or four times per second or so then it's in the bootloader and you have no sketch loaded.