Check if my AI is grounded

theres a couple ways you can do this and it partly depends on how u've setup your character if you were using a CharacterController to move your player this actually stores a value for .isGrounded so u can access ur charactercontroller and check that value, But given that you are raycasting im going to assume you don't have one on your character, below i'll paste a basic raycast designed for ground detection

         //this is our direction/vector that we raycast
         Vector3 downCheck = transform.TransformDirection(Vector3.down * 0.1f);        
         // this is just a copy of our raycast so we can see whats happening in the Scene.
         Debug.DrawRay(transform.position, downCheck, Color.green, 1f, true);
            // this is our actual raycast, it will use the downCheck to see if there is anything //directly beneath our character
    // if there is anything there it will set a boolean to true
    // otherwise it sets it to false
                 if(Physics.Raycast (transform.position, downCheck, 1f)) {
                         somethingdown = true;        
                 } else { somethingdown = false; }

edit -- Falling.

     // then lastly, we check the boolean(True/False) before we apply some falling this setup basically says if nothing is directly below us we will fall.
         if(somethingdown == false) {
               transform.position = new Vector3(transform.position.x, Physics.gravity.y * Time.deltaTime, transform.position.z);
         }
 
 

EDIT -- Revision of Code.

         Vector3 downCheck = transform.TransformDirection(Vector3.down * 0.1f);
         Debug.DrawRay(transform.position, downCheck, Color.green, 1.5f, true);
         /// Note this first Debug is just to see your raycast you can delete it after your sure its working right.
         if(Physics.Raycast (transform.position, downCheck, 1.5f)) 
         {
             
             somethingdown = true;
         } 
         else 
         { 
             somethingdown = false; 
         }
         if (somethingdown == true) 
         {
             if(dist <= distancebetweenobjects && dist >= Stop)
             {
                 mytransform.LookAt(Target);
                 mytransform.position += mytransform.forward * MoveSpeed * Time.deltaTime;
                 Character.animation.CrossFade("Armature|SoldierRun");
             }
             else
             {
                 Character.animation.CrossFade("Armature|SoldierIdle");
             }
             if(dist <= Stop)
             {
                 mytransform.LookAt(Target);
                 Character.animation.Play("Armature|SoldierHit");
                 
             }
         }
         if(somethingdown == false) 
         {
             transform.position += Physics.gravity / 10;
         }
     }
 }