How do you identify market opportunities for diversification?

I started an online writing service 5 years ago. It's a really competitive market and in as much as we've invested in designing a pretty good website and backend systems, our market has reached a plateau for a number of different reasons and we haven't grown much in the past year. We are now investigating the possibility of diversifying. As we have a solid team of people, writers and a backend system we can readily adapt to a number of other related services. Our weakness however is limited resources and inability to easily identify cost effective ways to acquire customers in our existing market and also with these potential new services. My question is: if you're diversifying into a new market area, how do you investigate whether the market is viable, and how do you find cost effective customer acquisition strategies? Nowadays it's relatively easy to design a good looking website, have good copywriting, and get Adwords running. But it's really hard (even if you have the people and systems) to acquire customers in a cost effective manner. I'd really like to know if anyone has been successful at this, and if they can offer any advice.