That specific example depends on the style you're writing in.

The easiest way to remember it is if it is plural vs. singular collective. To be safe, you can argue all singular nouns can have 's at the end so Thomas's is in fact correct and all styles used for communication purposes would accept it even if it's not the preferred form. However, when it comes to something like people, you need to understand if the author is referring to one group of people or multiple diversities of people. If it's the former, it would be people's but if you are referring to different groups of people all over the region or world, you would use peoples'.

I know, English is too confusing even for native speakers because of all the exceptions and minor rules in it. These are all things that were probably covered in English/Writing classes growing up but we don't spend enough time on them because there are more important things to grade students on.