Systems and methods for detecting ammonia slip in an exhaust system based upon transient NOx sensor responses are described. In one example method, an exhaust system allocates tailpipe NOx sensor output to NOx and NH3 levels responsive to the transient sensors using a segment length method that processes the transient signals based on the total segment lengths calculated within a window. A ratio of segment lengths relative to a threshold is determined for a measured and expected NOx rate of change downstream of an SCR that is further used to determine a probability of NOx and NH3 based on the measured sensor activities, and a controller is included to adjust one or more parameters based on the allocation and changes of sensor output.