Just because someone knows sign language does not mean they're qualified to interpret. While there are good interpreters who are not certified, you can feel pretty confident that you have a qualified interpreter if they are certified. A 3 is considered the minimum to be qualified to interpret, so keep this in mind when considering your needs. Someone with RID certification is a skilled interpreter. I'm not going into the pros and cons of each test here, it's just important that you know what the interpreter's certification is and to be sure they are qualified to do the kind of interpreting you need.
Some states prefer one test, some the other, and some have their own screening.
NAD and RID are working together to develop one test. Many areas have referral services. If there's not one in your area, see if you have a state commission for deaf and hard of hearing. They should have a list of certified interpreters. If this fails, try contacting colleges or schools in your area.