Fastest way to check if an array is sorted

Question

Considering there is an array returned from a function which is of very large size.

What will be the fastest approach to test if the array is sorted?

Answer 1

Here is my version of the function IsSorted

public static bool IsSorted(int[] arr)
{               
    int last = arr.Length - 1;
    if (last < 1) return true;

    int i = 0;

    while(i < last && arr[i] <= arr[i + 1])
        i++;

    return i == last;
}

While this function is a bit faster than in the question, it will do fewer assignments and comparisons than anything has been posted so far. In the worst case, it does 2n+1 comparisons. It still can be improved if you can make a reasonable assumption about the nature of the data like minimum data size or array contains even number of elements.