It used to work nicely but there was a lot of work done on the forums and regretably we lost the search function.
As Zubaz mentioned use the site search function with google but the works well part is a little bit problematic. You do have to play around with the search string to stike a balance between being too specific and getting no hits and not being specific enough and getting way too many hits to deal with.
Also to be a bit more specific if you're on the Sins site then you need to use "site:forums.sinsofasolarempire.com". In google you can also go into Advanced search and use the form there. I found that a bit more obvious the first few times I did it.
You do need to understand a bit about Stardock's forum structure. Stardock has 12 different forum sites
http://forums.demigodthegame.com/
https://forums.elementalgame.com/
https://forums.galciv2.com/
http://forums.impulsedriven.com/
https://forums.joeuser.com/
https://forums.politicalmachine.com/
https://forums.sinsofasolarempire.com/
https://forums.stardock.com/
http://tgnforums.stardock.com/
http://forums.vistathemes.com/
https://forums.wincustomize.com/
http://forums.xpthemes.com/
and over 750 !!! forums on these 12 sites all of which access the same database of threads. Of these 350 forums or so are unique but many of these are duplicated on multiple sites. For example you may think you're addresing only the Sins site but for example this Off Topic forum is visible from the Demigod, Elemental, Galciv2, ImpulseDriven, Sins and Stardock sites. Interesting enough this isn't one of the most accessible forums. Actually the PC Gaming and Personal Computing forums that are subforums of the Off Topic forum on most of Stardocks "game" sites are both more widely accessible and in fact on some sites are major forums with multiple subforums under them. In any case having some idea of the forum structure gives you some idea of who your audience is.
Needless to say this forum structure is quite complex and the work that was done on the forums was done to make all of these sites consistent. It really was a lot of work and I feel an overall benefit. All in all if losing the search function wsa the price of this then it was worth it.