Well, I think it starts with your objective and clarity of requirement. Then the second thing is to find leading vendors of that space. Make a list of top 3 (or 5 vendors). And then the most complex part comes- the comparison. Also, cost matters an important factor. Cost can change your requirement factors. 

First of all I assess what I need. Then I start looking for different opportunities and schedule calls with the companies to have a discussion about what they could provide with and costs. As soon I gathered all the information, I make a comparison on the different solutions, pro and cons for each oh these. I look for external feedback if available. Finally when I have the clear picture I can select the one fitting to my needs.

The Internet is a great resource. Additionally, most vendors that have made the grade on Gartner or Forrester or other advisory firm will have free reprints if you do not have access to any of these advisory firms. Belonging to various professional organizations is also an option to get opinions from peer-level individuals who may have researched or otherwise installed the product. Finally, always get recommendations from current users - albeit the vendor may be selective in providing access to just their happiest clients!

I do it like I always have done - I begin with clearly defined requirements of the capability needed, time constraints, etc. that the end user has.  I understand what is needed, how they will do what they need to do and the ergonomics involved in that, and anything that could be thought of as a negative feature.  Then I go do my research on the web, with vendors or wherever.  Before buying, we do extended demos to ensure a good match.  This seems to work very well.

Have you checked Cint.com? Cint has an Audience Insights exchange that connects you with panelists to answer your surveys