Pattern recognition, classification: how can one determine the probability of a correct result?

Question

Image processing

Actually there is a primitive program that should classify images into several categories. The Bag-of-visual-keypoints method and SVM classifiers were taken as the basis. And everything would be fine, but they asked me to make the program say how likely it is that this image belongs to this particular class. I still have no idea how to do this, therefore I ask you,% habrauser%.