One of the 'buzzwords' that comes up a lot in the ASF's discussions about open source projects is the idea of a community as being healthy for a project. For instance, the Tcl core has a healthy developer comunity involved - a group of people that actively participate, contribute and help one another, vet ideas, and so on. Other famous projects like the Linux and BSD operating systems have similar developer comunities, as do the Apache web server, Postgresql database, and many other examples of free software.
On the other end of things, we've all seen the sourceforge projects with no code, but just a web page and mailing list. In between, there are a lot of projects that have only one maintainer, and not much of a comunity effort.
This page also mentions the community aspect as being important: http://www.dwheeler.com/oss_fs_eval.html
The OSS/FS project site serves as a focal point collecting the improvements of various users. Thus, the project's Internet presence can give you an indication of how well the program is maintained. Examine the developer mailing list archives - is there evidence they're actively discussing improvements to the software? Are there multiple developers (so that if one is lost, the project will easily continue)? If their version management information is accessible to the public, take a look - are developers regularly checking in improvements and bug fixes? Freshmeat has a "vitality" measure that may help. Some OSS/FS programs are posted and then never maintained; depending on such programs is far riskier than depending on OSS/FS programs that are actively maintained. In general, is there evidence that the software is under continuous development, or has work halted?
Unfortunately, a number of Tcl packages, although far from abandoned, could probably use some help in this area: