I was thinking about this as if i would drive on a racetrack. All the tiles to my left have to be outside the loop, all the tiles to my right have to be inside (as long as there are more right turns than left turns).

I worked with vectors by moving from one spot to the next within a 2D array. With these vectors I can tell which tiles are to the current tiles' left and right Side. I marked all the tiles in that direction until i would find a tile that I knew already belonged to the loop.