how to detect if an entity is in front of a block

This is something I've wanted to figure out how to do for a while now for my mod Create: Obsidian. The mod includes flare entites and I need to detect if they are in front of my "Flare Detector" block I don't care about how far away the flare is, but in short: If a flare, at any point in time, passes in front of (is at the y coords of) The front face of the Flare Detector block then an action will run. I appreciate any help and look forward to a reply

-Cluckcoop