I would suggest identifying an author that writes what you enjoy and using the link in their profile that says find more posts by this author or find threads by this author. There are many writers that write a variety of subjects, however there are still other authors that write similar themes throughout most of their threads. I guess it depends also on the specifics of what you're looking for. For example the method I am suggesting would also be affective if you are seeking a particular writing style such as detailed, or plot driven. For example I myself write longer more detailed posts and this carries through to almost every rp I have.
You might also make sure you are in the specific forum your topic would be in. For example if you are looking for a thread that focuses heavily on incest or bdsm, I wouldn't go looking in the general original roleplay section for it, because those threads are generally put in taboo original roleplays.
Another tip would be to look at the authors of the thread cause sometimes that can help you narrow it down for example if you are looking for mxf or fxf. There are some here who play opposite genders than they are in real life, however from what I've seen still more write as their own gender. So if you saw "Life's journey -- (The gladiator and darkangel76)" and you were looking for fxf you could potentially skip that thread. Especially if you knew about the players in question like the example above both Myself and DA typically play as our own gender, therefore making this thread a mxf story.
Over all I can say is worst case scenario you read the first few posts of a thread, in theory the main characters should be introduced in those few posts, which should give you an idea of the pairing taking place within that thread. Take for example the thread title I mentioned above "Life's journey" In DA's first post she introduces Rhonwhyn, her priestess character. Immediately following that post I replied introducing Taurean, my assassin half-elf. So, within those two posts you can guess that the thread is going to revolve at least in some part around those two characters. This means the pairing is priestessxassassin, and fxm. If this does not interest you, you can close the thread and try another one.
shiesh, I got a little windy here. I hope some of my suggestions help.
P.S. I know I'm not on staff or anything, I just hoped my ideas would be helpful.