It's pretty easy to find the region interaction entries in an uncompressed save file (they're plain text) and to see which ones are thralling/husking.
However, nobody knows how a particular biome is associated with a particular regional interaction index. DFHack does not have the memory mapping. I managed to get the 34.11 save game file format about 70% mapped and didn't find it. In particular, the part of the file defining the biomes and map REALLY doesn't have any good candidates, so I started to strongly suspect there's a crosswalk table between evil biomes and regional interactions somewhere in unmapped data. (Which is a nearly totally useless observation.)
There are only a few useful ways to find thralling biomes:
1. Walk around and wait for clouds, which you've been doing. (Or embark on various biomes, which is a little faster, but still very time-consuming.)
2. Figure out which regional interaction applies to a particular evil biome by observing it. i.e. this biome rains goblin blood and reanimates. Frequently there will be only one or two regional interactions in the save game file that have that distinct weather. Then copy-paste (using a hex editor) the text for a regional interaction that has thralling/husking weather.
Since method 2 involves editing a save game file by hand, that's pretty horrible.
It's also probably possible to do method 2 using dfhack, but I haven't figured out the specifics there.
So, to answer your question, there's no known easy way, and people much better/more obsessed than me (like Quietust) haven't found one. (And I routinely move biomes and rivers around to make niftier embarks with 3-4 different evil weathers and waterfalls/cliffs, which is also pretty obsessive.)