China Mobile sends out a reminder when your account reaches a particularly low balance, but these messages tend to get lost amongst all the other junk mail. If you missed it, don’t worry. There’s still hope! Write a text message that says Hfcx (and nothing else) and send it to the number 10086. A few seconds later, you’ll get a return message.
This new message is, of course, all in Chinese, but if you scroll down, there will be two numbers followed by a yuan symbol (元). The first number tells you how much you spent last month, and the second number is your current balance. Keep in mind, though, that I’ve only tested this with phones bought in Jiangsu province. The rules may be different in other parts of the country. Feel free to share them if they exist.