What are the criterias needed to work for UN/Internationa Law?

What are the requiremnents needed in order for you to be able to consult the UN in Geneva, Switzerland regarding International law issues and report to the headquarters in NYC? I know someone who claims to be practicing International Law who is a US citizen and is a consultant working in Geneva. Do you have to earn both license here in the US and overseas in order to practice? And is there a way to check the legality of such lawyers working/consulting for UN?