One of the purposes of this thread is to pick up suggestions for improvements...
I haven't played DF before 0.40.X, so I have little idea of what they looked like. I don't know off hand how to locate volcanoes, but it ought to be doable. Magma pipes can probably be located, but I don't know how precise the info on depth is. A guess would be that the info is what the highest cavern reached is, rather than an actual depth in Z levels.
A true designated volcano I would think would be like finding streams or other map level objects since they show up on the world map themselves.
Non designated volcanoes and instead looking for high magma would be the hard one. Caverns and magma aren't always linked it appears. Some of that may be the weird world gen parameters I put in so needs validation from others. I can find magma below the 3rd cave system on some maps and above the first on others. Volcanoes can sometimes be way above the ground level and other times top out below it. Once in a blue moon it used to be possible to find one that opened at the surface level itself but that has gotten super special rare now if they even happen anymore.
Obsidian surrounds the magma pipes; that may be a detection method if the magma itself is hard to detect. That may be easier to target than the liquids.