For example, will it help people do more work or better work, will it help reduce the need for office space, will it help attract and retain top-quality people, etc. Also, try for a very modest experiment - ask if you can work as little as one day a week at home for a month. That is a commitment of only four days - not a big risk for anyone, but it's often enough to get you started and get past the initial resistance. It depends on a combination of the nature of the tasks involved and the technology. Also, look for jobs that are physically portable - that is, whatever the telecommuter needs to do the job can be taken home in a briefcase or box, or can be accessed via phone line.
Good employees make good telecommuters.
Someone whose performance is mediocre in the office isn't likely to do well away from the office. Look for people with demonstrated ability to manage their own time and workload well, solve many of their own problems, and find satisfaction in completing tasks on their own with minimal direct supervision. It's definitely important for employees to want to telecommute, and it's a bad idea to force people to telecommute. But wanting to do it and being suited and capable to do it are two different things. The best approach is to identify your minimum selection criteria and then encourage prospective telecommuters to learn more about the pros and cons of telecommuting.