How do I know if I used up all my cellular data ?

Many cellular carriers have an option where they will notify you when you have reached various percentages of your data allowance (e.g., 80%, 90%, 100%) so you may want to contact them to see if they offer that. Also many have a web site where you can go to see your current usage amount. The other option is to setup an automatic reminder on your calendar for the day when your billing period starts and each month when you get that reminder, on your phone go to Settings > Cellular and scroll to the bottom and then "Reset Statistics." If you do that then you can check usage on your phone at any time by going to Settings > Cellular and looking at "Cellular Data Usage - Current Period."

Also, many cellular providers have a number you can call from the phone to get an automated update on current usage.