Nature already does this. Why not simply look for the lastest impactor- say of size range of 5-10 meter diameter [crater somewhere around 50-100 meters]. Probably one on Mars which is several years old. A hard target to find though [unless a fresh crater has some characteristic which is perishable and very distinctive]. Instead you put instruments in orbit and/or on surface which could detect impacts- or maybe if you had a camera looking at a large portion of the nightside of Mars, you could pin point the impact site within a few sq kilometers and probably detect impactor size down to say a meter or two.

As far as Europa, it's probably got around the same impact rate.