Well first off you're going to need to know the ESN, MEID, or IMEI of the desired phone you need to check. The information below should help get you started.
As you see in the illistration to the left... the ESN, MEID (or IMEI) of your phone is generally displayed on a white sticker under the phone's battery. The ESN, or MEID will be in one of two formats, DEC or HEX, and is either 8, 11, 14, or 18 characters long, and the IMEI is usually 15 characters long.
Once you have the ESN, MEID or IMEI of your device, you'll need to select the provider you want to check against from the drop down menu, then enter the number of the ESN, MEID or IMEI in the blank field and click "Check ESN".
The status of your ESN check will come back in one of many. They range from cleanESN, badESN, and invalidESN. You may see the full definitions HERE.
Lastly, if you feel an ESN you are checking is coming back with invalid, or incorrect response, please feel free to contact the wireless carrier to verify the ESN directly. It is advised to call more than once to check an ESN!