From Forum Nokia Wiki

The amount of free RAM can be logged by putting the following code somewhere that is frequently called:

#include <hal.h>
 ...
   TInt freeRam = 0;
   HAL::Get( HAL::EMemoryRAMFree, freeRam );
   // If you want to trace RAM usage
#ifdef _DEBUG
   RDebug::Print(_L("MyDLL:FreeRam now = %d"), freeRam );
#endif

And the .mmp file may also need this adding:

LIBRARY         hal.lib

Use User::Available() for getting the total free space currently available on the current thread's heap and the space available in the largest free block.