Several of the existing answers on the matter of the IDP point to the advantage of it being in a Latin script and thus easier for foreign police and car rental agencies to verify. Generally there is also at least one answer saying you don't need one.
- What is an International Driver's License?
- Are international driving licences really necessary for any country?
But mingled in with all the YAGNI (especially for someone like me whose license is in English) is the recurring "it depends on the country" thread. So I checked what my government has to say and for some countries it says "an International Driving Permit is recommended, but ..." and goes on to explain that you can use your home license for some period of time, while for others it just says "an International Driving Permit is recommended." And then finally some say "An International Driving Permit is required." What even is the difference between recommended and required in this context?
And is this the last word on the matter? Is there some authoritative list I can consult? So far, of the 9 or 10 countries I plan to visit (depending on whether you count the Cook Islands as part of New Zealand or not) on this trip, I only plan to drive in 3 or 4: Vanuatu (recommended), Samoa (have to get a local one on arrival), Tahiti (required), and the Cook Islands (have to get a local one). If I don't plan to drive in Tahiti perhaps I don't need to get one.
[Update for those using this question as a source of information: Tahiti never once asked about the IDP and we did not rent in the Cook Islands but I have heard they no longer make you get a local one. Samoa still does, we did, and I even got to show it at a roadside check.]
(Yes, the prudent thing to do would be to go get one. But it can only be done in person and it's at least an hour away so for both of us to troop off on this errand to obtain an unnecessary piece of paper just bugs me. As a result, I would like to know how to know so I don't waste my time on a fools errand.)
Not tagging with countries because the point of the question is how to settle this in general, rather than getting the details of any particular country.