Get yourself a copy of the Writers Handbook, or the Writers year book and a highlighter pen. Pick out publishers and agents who handle what you write, then do some additiona research on them.
Check out their website if they have one, have a look for them on the background checks forum here.
Follow submissions to the letter.
Also, as others have said, look at who publishes the sort of books you like to read, have a wander around waterstones to see who else is publishing those books now. Also, it's always worth seeing what the trends are. For instance, if one publishing house in particular currently has a lot of nano-technology inspired stories on their normally mixed list, then they will probably not want to buy another one(bear in mind how long it takes from acceptance to print)
That said, there are some publishers who ONLY publish a certain type of book. There are publishers who only publish paranormal romance, or sword and sourcery.
The main thing here is to do your research.
agentquery is a great site. And free.