That depends on if you are using a comcast access point or not.
If you are, you can navigate to http://192.168.100.1 default login is user: admin pass: admin (or password) cant remember.
From there, assuming wifi is turned on, you can navigate to look at connected devices. I am not aware this will show you how much they have used however. You might need a more advanced router for that.
FWIW their gateway devices use http://10.0.0.1 for access to the user interface pages. AFAIK, there is no usage counter. Also, it may be listed as *attached devices* or *DHCP clients*. The default password is password.
As EG said, when you put in your username and password on the Comcast gateway (Default username: admin
password: password )
on that very first page you'll see in orange :"Connected Devices" which will show any device connected to your wi-fi. Click on "View Connected Devices" for more information. You obviously should not see more devices than you have in your household. If you click on "Edit" you'll be able to name each device. If there's something you don't recognize just click on the "X" and it'll be gone.
If this is a problem change your network key password by clicking "Connection" and then "Wi-Fi" from that home page. There you can click on "Edit" and change your network password to keep them out.
There is no way with the Comcast firmware you can see the usage from each device.
And if MAC addresses are also displayed, you can cross reference them with the MAC addresses that are on the labels of the devices that you actually own.