Check your pre-paid balance
If you use a pre-paid SIM, you can check your balance at any time by sending an SMS with the text "ye" to the number 10086. In a moment you should receive an SMS reply with your pre-paid balance.
Check your data usage
If you have a data plan, you can send an SMS with the text "1091" to 10086, and you'll receive a reply stating how much data you've used and how much is remaining.