If you are going to invest in a domain name, you want to ensure people can find it not just from direct traffic, but through search. Beyond finding your site, search is also a powerful tool in that it can show you how popular a search term is in a certain language.
Google AdWords or any pay per click advertising service can give you a sense of monthly search activity for your potential IDN. Also, do a little research on the countries you may be interested in purchasing an IDN for to see their needs, trends and most popular sites. What does the word mean in their language? What does it mean in ours? What did the .com sell for? You can also give a call to one of representatives for some advice and guidance on how to choose the best IDN or group of IDNs.