You need a geo survey to see it. If you have already surveyed the system and you use the SM functions to put a ruin on a planet then you will need to update the screen to see it. You will see a general statement about the size of the ruins. ie ruined settlement, intact cities, ect. Generally the order from smallest to largest is: Destroyed, ruined, abandoned, partially intact, and intact. Within that is a descriptor of: outpost, colony, settlement, installation, city. The largest can have several thousand installations to attempt to recover. Just remember to have some guards allong to protect the engineers.
Brian