Getting an accurate read on workers' feelings toward the union is the difference between winning and losing an election. An accurate assessment of workers' intentions can tell an organizer whether to proceed with filing a petition for election, which workers to target, tactics and strategies to employ and how to allocate resources. Such contact takes time and effort but improves accuracy and objectivity.
Telephone calls are faster but are less reliable in assessing a worker's support.
Word of mouth isn't reliable and should be avoided at all costs. I like to look people in the eye and read their body language. I think people will open up and tell you a little more in person than they would on the phone. The key to assessing workers' intentions is listening. Organizers shouldn't overstate or suggest what a worker is saying.