you cannot perform an accuracy assessment without external information.
You have created a result. The only way to find out how good it is, is to compare it with an independent reference. There is no information about accuracy within the result itself.
Collect points in Google Earth, for example, label them as urban or non-urban. Then import them into SNAP and check how many of the urban points are classified as urban in your result. This in relation to all urban points is the true positive value. The points which are labeled as non-urban and are non urban in your result is the true-negative value. And so on…