I used Topo Reader that has amazing library of all years of maps that allows easy use of overlays and marking old buildings to check corresponding current map for removal or still intact. Any others?
For a second I thought you meant the "metal detector apps" that supposedly detect metal. Haha. I didn't even know there were apps that would help (never thought to look). I'm checking out that Topo one now.
EDIT: Can you link to the app? I'm not finding one like the one you described.
I just found what I think to be the coolest app ever for detecting. It's called Maprika. It is in the playstore. I ran across this on YouTube. The title is "How to use GPS on old maps with your phone" by task force history. You need to watch the video. This thing is so cool. It's an app for ski routes but can be used with all kinds of things. The guy in the video does a better job of describing how it works. Basically let's you overlay google with old maps. That way you can see where you are but on the old map. You can see things like battle lines or earthworks. Really cool. Go see the video.