Finding the order of sorting of an array

A few tips; firstly, localise loop variables. i.e. Instead of

int i;
for (i = 0; i < len; i++)
{
    scanf("%d", &input[i]);
}

Use

for(int i = 0; i < len; i++)
{
    scanf("%d", &input[i]);
}

This is mostly a readability thing as it makes it clear what the variable i is used for. (Make sure to enable c99 mode on your compiler though)

Secondly, don't use 'magic numbers'. This is another point for readibility, but it can make writing the program much easier, and for longer projects, it can be a huge help on debugging. Also, if you change it's value, you only need to do it once and it's changed everywhere. i.e. just after the header files add something like

#define NONE 0
#define INC 1
#define DEC 2

And then in your function, return these symbolic values instead of the actual values.

Thirdly, I'd use a set of 'if' statements instead of the switch in your main function as you only have 3 possible outcomes. For example, I'd use

if(order == 1)
    printf("Sorted in increasing order\n");
else if(order == 2)
    printf("Sorted in decreasing order\n");
else
    printf("Unsorted\n");

Fourthly, I noticed you added a user input for the length and used this value in a malloc(). You should change len from an int to an unsigned int to prevent any attempts at allocating a negative number of bytes. You also assume that the malloc() will be successful. You should add a check for that, and some error handling for if it fails. i.e.

int* input;
if( !(input = malloc(len * sizeof(*input))) ) {  //If the malloc() fails
    fprintf(stderr, "Memory allocation faliure\n");
    return 1;
}

Here is a somewhat rewritten version of your code with comments explaining all the changes I made that I didn't mention above:

#include <stdio.h>
#include <stdlib.h>

#define NONE 0
#define INC 1
#define DEC -1

int find_sort_order(int array[], int len) //I renamed your function because caps in names are a bad idea (debugging issues)
{
    int order = NONE;
    int order_prev = NONE; //Compare this to order at the end of every increment to see if the order changed

    if(array[1] > array[0]) //Determines the initial order of the array for comparison later
        order_prev = order = INC;
    else if(array[1] < array[0])
        order_prev = order = DEC;

    for(int i = 2; i < len; i++) {
        if(array[i] > array[i - 1]) //Determine the current order of the array
            order = INC;
        else if(array[i] < array[i - 1])
            order = DEC;

        if(order != NONE)
            if(order != order_prev && order_prev != NONE) { //If order was not NONE initially and it has changed, set order to NONE, and break out of the loop
                order = NONE;
                break;
            }
    }
    return order;
}

int main()
{
    unsigned int len;
    int* input;

    printf("Enter length of the array: ");
    scanf("%d", &len);

    if(len < 2) {
        fprintf(stderr, "Can't have an array of of less than 2 elements!\n");
        return 1;
    }

    if( !( input = malloc(len * sizeof(*input)) ) ) {
        fprintf(stderr, "Memory allocation faliure!\n");
        return 1;
    }

    for(int i = 0; i < len; i++)
        scanf("%d", &input[i]);

    int order = find_sort_order(input, len);

    if(order == INC)
        printf("Sorted in increasing order\n");
    else if(order == DEC)
        printf("Sorted in decreasing order\n");
    else
        printf("Unsorted\n");

    free(input);
    return 0;
}

NOTE: I removed as many of the braces as possible as I thought that they made the function look bloated, and my personal preference is to use indentation only for 1 liner 'if' statements, but that's entirely up to your preference.

EDIT: I noticed a couple of small bugs in my code causing odd behaviour and have fixed them