how will we able to know that we will get the image behind the mirror

see, if the image  distance comes out to be positive, by using  mirror formula then image will always   form behind the mirror and it will always virtual. and if image distance is negative then  image will always will always  real and form in front of the mirror.

Now  in case of the convex mirror, image will always form behind the mirror, irrespective of the position of the object, while in case of the concave mirror  if the object is  placed between pole and focal point then  only image distance will come out to be  positive and hence image will form behind the mirror.