Q. As missionaries transitioning from one field to another, how to identify which UPG's are unengaged, or to put is another way, how do we get up to date information on the progress in any particular UPG?
For broad information ("identify which UPGs are unengaged") - the first places to start would be the big global repositories of information for the unreached - Joshua Project, Wycliffe, the IMB, the Center for the Study of Global Christianity (World Christian Database), etc.
However, mostly all these groups will be able to initially tell you is whether the group is engaged or not - they may not have a great deal of information beyond that readily available.
For up to date information on any particular UPG - I will tell you the process that I use for compiling a Cluster Survey. This is probably the most direct process, and it typically takes me about 30 days.
The first thing to do is to try and find someone working among a particular group. The bigger the group, the easier it is. I usually start by trying to figure out what affinity block and cluster the group is in, and then ask myself who of the "big" groups is most likely to have a worker there. Then, I just start emailing and calling around. I usually am looking for someone who has spent 10 to 20 years of their life working with the people group. If that's not possible, then I look for someone who's spent a large amount of time, or for an agency that's had multiple people over a long span.
Once you have found a worker (of any sort), the next step is to ask them for an interview. Explain the following:
Once they agree to the interview, set up a time, and have the method for the interview ready. I usually use something like Zoom, which allows me to record the interview (but I also type 120 wpm so I can take a pretty good transcript as we're talking). Conducting an interview is a skill to be learned, but I can give you some broad outlines:
Last of all - and this is key - ask them for the names of other people who you should talk to. Ask if you can use their name as a reference. Emphasize security. If you've been gracious and attentive and a good listener--and they know someone--you'll probably get a reference.
I usually wash, rinse, repeat this process 10 to 15 times. Somewhere around interview 12, I usually start hearing the same things I've already heard--and at that point, I know I've found out pretty much everything that can be found out by interviews. Then it's just a matter of compiling it all together.