This thread is rich with information:
https://forums.frontier.co.uk/showt...Fumarole-Location-Catalog-and-Data-Collection
If you find something, be sure to make a contribution to that thread.
In short, it's mostly good luck. Without advanced scanning tools that map geological features of landable bodies, we must rely on the old method of flying over the surface until we find something. It is thought that one's luck in such an endeavour is increased by choosing to search small bodies, preferably 500 km in radius or less. On these bodies, you want to fly along the canyons. These canyons seem to have a higher incidence of geological activity than flat plains, mountains, or craters - although it is possible to find fields of geysers and fumaroles in craters, it is rare. There is an explorer's superstition that your luck is increased by searching between 30 degrees latitude North and 30 degrees latitude South. I have definitely found fields of geysers outside this band, but I will admit to searching mostly within the tropics. It is easier to find iron magma and silicate vapour volcanism than water or carbon dioxide geysers or water magma geysers. Icy bodies tend to be scarred with a huge number of faults and canyons. Being able to choose one of only a few canyons on rocky worlds seems to help. Good luck in your search!
My own personal white whale is to find fumaroles on rocky ice worlds, like the one found in Actaea in the Sol system. I have never found those particular structures anywhere except Sol. Even though the description for rocky ice worlds says that they are prone to volcanism, in practice they rarely do. On the few occasions that I have found rocky ice worlds with volcanism, I have only found regular water geysers.