1. You can tell them apart by the structure that holds their seeds. In the case of the Norway Maple the seeds are held in a winged "samara" (seen in the image above by the left leaf). In the case of the London Plane the seeds are located in a ping-pong sized ball (see image below).
2. You can tell them apart by their bark which is very different from each other. The first image below is of the Norway maple bark.
The next image is of the Londan Plane bark on a mature tree that shows the characteristic lumpiness and bulging. The bark peels off in small sheets, which is one of the reasons it does so well in high pollution cities.