copying OSM data

Are there any official guidelines of what to do when data similar to OSM’s data is spotted?
I’ve noticed data that’s not exactly identical, but quite similar to my contributions on several websites (seems to be the same source).
How to make sure it’s not my imagination playing tricks on me? Is there any program to compare map tiles?

There is some information in the wiki about this subject.

Thanks for the link. I don’t think I should follow that, since I don’t want to accuse anyone of copying without hard evidence. These guidelines assume that I’m certain about that fact.

The data is only available as tiles, so I can’t just compare lon/lat values and say “hey, it’s copied!”.
There are some hints - a few roads that are more like simple passages, invisible from satellite photos and not very visible on the spot, but they still could be done by somewhere else.

According to that wiki page, I should now send a polite email explaining them the situation, but should I send an email saying “Your data might have been copied from OSM”?