How can I find out a researcher's current affiliation? What is the best way to find out a researcher's current affiliation? Let's say I know a name and a field, but not much more. 

A google search seems like the most obvious approach, but unfortunately university personnel or research group pages often seem to be quite poorly maintained, and it is not uncommon that someone is listed even if they left more than a year ago.

Early-career researchers are particularly difficult to track down since they often have less of a "presence" on institution webpages and are likely to be moving relatively frequently.
If contact details of a specific student/postdoc are hard to find, contact details of the supervisor or group leader are likely much easier to find, and you can always politely ask him or her as to the whereabouts of the student/postdoc.