Typically speaking, CWI's are qualified to perform Visual examination of welds. Interpeting Xrays (and gamma ray RT film) "may" be within a CWI's capabilities but these should not be considered a "given."
Performing PT testing is usually required to be performed by an ASNT SNT-TC-1a NDE inspector although once again, the CWI may be capable (thus qualified) depending on what, if any codes are invoked.
The quik and easy answer is to look for someone with the required experience per ASNT TC1A for RT and PT who also has the CWI. However; even in that, you need a program to support that. If you are starting from scratch, It may be a good idea to look for a Level III in the methods you require who also has a CWI cert or better. That option will cost you more, but that individual is qualified and certed to create the program if they have relevant experience.