TileMap 2D - Automatically placing specific Prefab on Corners

Im creating a TileMap generator and im a beginner in Unity.

Currently it works to create a big Block 50x50 with Grass Textures. Now I try to figure out how I can make "round corners". I allready created new "half-filled" Textures for new prefabs to place them in the corners.

Now need a way to check If there is a block above, next to, etc... the current position.

My goal is something like this: http://answers.unity3d.com/storage/temp/63358-test.png

And this is my Code, can anyone help a noob?

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 namespace Tile {
     public class TileEngine : MonoBehaviour {
 
         //TODO: Remove
         public void Start() {
             CreateTileMap(new Vector2(50,50));
         }
 
 
         private Vector2 mapSize;
 
         private Tiles[,] tileMap;
 
         public GameObject grass;
 
         public void CreateTileMap(Vector2 mapSize) {
             this.mapSize = mapSize;
             tileMap = new Tiles[(int)mapSize.x, (int)mapSize.y];
 
             for(int x = 0; x < mapSize.x; x++) {
                 for(int y = 0; y < mapSize.y; y++) {
                     tileMap[x, y] = Tiles.Grass;
                     InstantiateTile(tileMap[x, y], new Vector2(x*0.25f, y*0.25f));
                 }
             }
 
         }
 
         private void InstantiateTile(Tiles t, Vector2 position) {
             if() {
 
 
                 if(t == Tiles.Grass) {
                     Instantiate(grass, position, Quaternion.identity);
                 }
             }
         }
     }
 }