If you are on the Vodacom mobile phone network then it is a pretty simple task to check your remaining airtime if you are a prepaid (or even a contract) customer via their nifty little balance enquiry feature.
For Vodacom prepaid subscribers, a quick dial to *100# (and similarly for contract subscribers, a dial of *111#), will give you an update on your remaining free minutes or SMS bundle. (Follow the resulting menu options if applicable.)
Note that this service is completely free of charge. How nice of them.