Great answers by experts there.
1. How they communicate their own branding and story (for the services that they provide)
2. What kind of insights and experience they share (their blog, community publishing platforms, social)
3. Their core values and work culture (it reflects in their online presence, if not explicitly communicated)
4. What questions they ask you (asking right questions is extremely important; it tells a lot about their approach, work process, and their experience)
5. LinkedIn profiles and twitter accounts of their key team members (shows their social score though this is not a parameter to evaluate their skills; it helps you know them better)

Any more questions, setup a call for me and I can give you more details! :)