Your question touches upon a much more fundamental issue, which is the actual objectively measurable benefit of development aid. Many conservatives and plenty of those having worked in development for extended periods of time seriously question the potential impact of development aid. The former out of principle, the latter through personal experience. And, over the last few years, the general opinion appears to slowly be moving more towards a consensus that the only type of aid that has the potential for some structural success is emergency aid (following earthquakes, flooding and the like).
And this is not without merit, at least on the face of it. Think about it, outside of emergency aid, every volunteer, or even every development worker's position, in the end, takes up a job that could and perhaps even should, have been filled by a local. Jetting in some outsider, to jet them out a while later, is not sustainable.
If local skills are lacking, bringing in the outside specialist is not suddenly going to create local specialists with the same educational basis, which apparently was required to create the specialist in the first place. The only structural solution, then, is to work on the education system.
But, to go even further, perhaps your implicitly stated objective, of 'finding the good guys in international aid' should not be your focus at all. As the Peace Corps, arguably an organization that at least has no direct financial objectives with its work, wikipedia page states, their mission is threefold:
- providing technical assistance;
- helping people outside the United States to understand American culture;
- helping Americans to understand the cultures of other countries
Not at all focussed on delivering 'aid', but, primarily, focussed on building a cultural bridge between the host and American culture, fostering understanding and also perhaps why an organization like IESC is generally reasonably successful; those sent abroad through IESC are doing it because they like the challenge and don't care much about the financial consequences, except for the participants typically wanting to optimize the relevant tasks, financially and organizationally, for reaching their objectives.
In other words, if you can find an opportunity that brings you satisfaction, for whatever reasons you value, it's probably a good choice.
Now, if you worry about the financial integrity of any organization operating in international development, immediate impact might be relevant, but not long term sustainability, meaning that, in the end, immediate impact, also, is irrelevant, unless it's part of some emergency aid package.
No program that flies 'volunteers', let alone 'paid specialists', from one side of the world to the other, for a short period of time, to put them in a social bubble where they have no clue about the local norms and values, can have any long term positive impact.
So, perhaps more directly answering your implicit question, the only structural long term benefit can be had through emergency aid, though even there it's not always easy to separate the fortune seekers from organizations that try to make a real change.
But, here, as for any 'regular' commercial business, opaque organizational structures and lack of independently verifiable success are good indicators of a sub-optimal organization. In other words, if you can't do your due diligence, the organization is probably hiding something.