First step would be to try the Telkom ADSL line speed checker tool:
How to check your available maximum line speed
Checker Tool. Use the Telkom ADSL line speed checker to see what speeds are available in your area.
You could also try looking at your modems reported max speed under status/statistics - but not all modems reports this and this is only an estimate and does not represent the exchanges supported speed.
Else you can give Telkom a call and find out what speeds your exchange supports.