*3 will announce your balance and let you make a payment over the phone. Use *4 (or *5 for Spanish) to hear full balance info and usage. There's no number on Sprint to get just a text message, like there is on ATT, but *4 does give you the option to have the info you just heard, sent to your phone.
It would be so easy for Sprint to offer a simple no nonsense Android app to display this same info...I'm not sure why it hasn't been done yet. I think it exists, but for a limited number of devices only, not including mine.