how to detect when a signal overlaps other on a CRO screen

how can i know whether a signal has overlapped on other on a C.R.O.

for eg. i want to know whether a circle has overlapped on a semicircle on a C.R.O

actually my circuit will make a circle move continuously on the C.R.O screen . and a user will be moving a semicircle thru his control keys horizontally and vertically.how can i know when these two touch each other ???

This cannot detect on the scope screen electronically.
Detect it in Your circuit.