Various systems and methods are described for detecting ammonia slip. In one example method, an exhaust system with two NOx sensors uses transient responses of the NOx sensors to allocate tailpipe NOx sensor output to NOx and NH3 levels therein. An ammonia slip detection counter with gains is included that determines a probability of NOx and NH3 based on the measured sensor activities that are further processed by a controller to adjust one or more parameters based on the allocation and changes of sensor output.