A fog click is any click on an enemy hero when that hero should have been unclickable due to fog of war. It is usually a very strong indication of Map Hack, though there are a few instances/bugs where this is not the case.
To catch fog clicks, you need to select the player you are suspicious of from the drop down, turn off fog of war, and spam Ctrl + c (which will center the camera on the unit selected by the player). You can download a script/write one yourself to do this for you (search Case C script if you don't want to write one).
If you pause the game on a fog click and either minimize or turn on fog of war and turn it back off, the selection circle showing the fog click will disappear.
- At the start of the game, it often happens that the blue player is selected by enemy players. Known bug, not indicative of map hack
- When blue dies and respawns, the same bug occurs and enemies will seem to have fog clicked blue. Known bug, not indicative of map hack
- If a player clicks an enemy right before they are fogged, the selection circle will sometimes show up in the replay after they are in fog. Known bug, seems to be the one that is confused in the FM replays if I'm reading these posts right (haven't watched the replay myself, but this is a bug I've encountered when trying to find maphackers).
In most other instances, a fog click is map hack.
Note that maphacks have gotten to be more sophisticated. Some sport something called "anti click" protection. If I understand correctly, whenever the player using maphack clicks an enemy hero, the maphack software will prevent the packet from being sent out that has the fog click action, meaning it won't be in the replay data. Of course, there are still things you can look for in the parsed data that can indicate something like this is happening, but it is more difficult.
Lastly, some maphacks simply have player colors on the minimap. It can be very time consuming to catch a "smart" maphacker, and can require hours of perusal over a replay to catch the clues that give them away.
I've got several replays of ban requests I've submitted in numerous communities including PDIH of players caught and banned for using some of these more difficult to detect maphacks, and I might still have the log files I created of the timestamps I used as evidence against them. If you are interested and PM me on the forums (I am not able to check the forums as much as I would like these last few weeks), I can provide the replay and timestamps for you so you can see what exactly it is you need to look for.
I'm trying to adhere to the forums rules about discussing map hack and can't get into any further detail. I think what I've posted is public enough, especially in light of all this recent controversy.