If all you have is the pointer to the first element then you can't:
int array[6]= { 1, 2, 3, 4, 5, 6 };
void main()
{
int *parray = &(array[0]);
int len=sizeof(array)/sizeof(int);
printf("Length Of Array=%d
", len);
len = sizeof(parray);
printf("Length Of Array=%d
", len);
getch();
}
// output: Will give two different values: 6, and 4.