Two roads diverged in a yellow wood (part 1)

JavaScript(ES6), 19 12 bytes

Edit:

A more golfed version is

a=>a.trim[1]

Returns # for right and a space for left.

Original:

a=>a.trim()[1]=='#'

Explanation

Ungolfed:

function(input) {
  return input.trim().charAt(1) === '#';
};

The first thing this function does is remove white space the beginning and end of the input. This means that the first character is always #. Then from there I check the second character(JavaScript starts at 0) and see if it is a # character. This returns a boolean. If the path is right it will be true, if it is left it will return false.

How I golfed it

In ES6 there is an anonymous function shorthand called an arrow function. This means that I can take my wrapper function and turn it into:

input => ...;

Due to the rules of arrow functions it will return the rest of the code. From there I converted charAt(1) to [1] as it is a shorter way, though not recommended. Then I took === and turned it into ==. While they are different in this case it doesn't matter. Finally, I renamed input to a and removed all whitespace.

Output right and left

While the puzzle doesn't actually need the program to output right and left, here's an example of other outputs:

a=>a.trim()[1]=='#'?'right':'left'

The only added part is ?'right':'left'. This creates a ternary operator, a condensed if statement, this means that the(ungolfed) code is equal to*:

function(input) {
  let output = input.trim().charAt(1) === '#';
  if(output) {
    return 'right';
  } else {
    return 'left'
  }
};

Example

// Function assignment not counted in byte count
let f =
a=>a.trim()[1]=='#'

<textarea placeholder="Put path in here" id="path" rows="10" style="width:100%"></textarea>
<button onclick="document.getElementById('result').textContent = f(document.getElementById('path').value)">Submit</button>
<p id="result"></p>

CJam, 1 byte

r

r puts the first string of adjacent non-whitespace characters from STDIN on the stack, so this prints ## for left and # for right.

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Pyth, 2 bytes

hc

Outputs # for left and ## for right.

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Explanation

hc
 cQ     Split the (implicit) input on whitespace.
h       Get the first part.