<p>One problem I have with open-source is not often the product or documentation, but the level of community involvement and support. Response time for some questions on official forums can take more than a week, which can honestly make a big difference when choosing a platform for commercial services use. However, I find that evaluating the community beforehand is very difficult. How do I know what are the most common issues, and whether or not people are getting support for them? How do I know what problems I personally might encounter based upon my project, which would require some level of community support? Do you have a formula for performing this evaluation?</p>

If your question is posed because you don't want to pay for support, then I think your best bet is to ask questions about a specific project on neutral forums like this and see what others have experienced in terms of support and timeliness of resolution to issues.
If you are willing to pay for support, there are companies that provide commercial support for open source projects, like
OpenLogic
as an example that could factor into your formula.