proper pseudo-potential for a system

Hi, I am a beginner in using siesta code. How to decide the r-core value for *.inp input file for pseudopotential generation by ATOM program? How do I sure that the generated pseudopotential is good for my system. For example, suppose I take GaN system, then how do I sure that the generated Ga.psf and N.paf are good enough for describing the electronic structure properly?
 In Contrib directory I find some provided pseudopotential, are they good enough to use??