Identifying vehicles and objects in the vicinity of platooning vehicles is critical for platooning safety. A neighboring vehicle may change lanes and enter a lane with platooning vehicles. Any deployable platooning system must be able to first identify the presence of a cut-in vehicle before performing any control actions on the vehicles. In this article, a sensor fusion algorithm is presented that combines radar and vision data obtained on-board a moving truck to identify the states of the cut-in vehicle. Experimental results are presented to corroborate the performance of the proposed algorithms.