Method one.See from the flash RPi LED
When the raspberry pi work, You will see a red LED lighted and a yellow LED flashing.
If you just see the red LED and the yellow LED not flash,You need to reburn the Raspbian OS or try to change a new SD card:
Method two.With a Raspberry Pi LCD
Generally You should see the following picture after the right boot:
If you get this following picture or similar phenomenon
You should reburn the Raspbian OS to your SD card