How do I know if a PhD is the right choice? I am graduating in September in Economics from a world top school and I am willing to go on studying to earn a PhD (in Economics). My Master program is very focused on quantitative skills and research and although professors say we would be able to work in any field because of our strong preparation, I am feeling that only a PhD is the right way to make my studies worthy. 

I decide to study Economics with the purpose of becoming an economist in a supranational organization and I feel I have found my field of interest too. My fear is that I am feeling pushed to enroll in a PhD because it seems to be the natural continuation of my studies.

Thus, without referring to my specific case, I was wondering if there are some signals, something objective, that would make me able to understand if a PhD is the right choice for me, besides the fact that I am feeling interested.
If your goal is to actually do research in economics, having only an MA will be severely limiting.

It's possible to get research assistant/associate positions at important institutions without a PhD, particularly if you're willing to work in the non-profit sector (think tanks, foundations) or in related areas like banking.  However, having been at two very prominent ones myself (Brookings and Urban), there is definitely a glass ceiling that even skilled researchers with a track record of successful research struggle to get past without PhDs.

However, it sounds like you're more interested in larger policy groups, like the IMF, World Bank, BIS, or central banks.  It will be even more difficult to get in the door there without at least being a PhD candidate.  These places generally have more full PhD applicants than they can handle, so they face little incentive to consider non-PhDs for anything beyond supporting roles.

In short, if you're willing to compromise where you work and potentially face a glass ceiling eventually, you can definitely do economic research with an MA.  Otherwise, a PhD is entirely necessary.