Q: How do you know that an interpreter is qualified?
A: 1) Visit the Registry of Interpreters for the Deaf link below,
2) Click on Search Tools at top of page,
3) Click on Find An RID Member,
4) Check that they are listed as Certified under Category.
If the person is not Certified, then you have no way of knowing if they at least have the minimal skills required to produce an equivalent interpretation.
Please click on the links below for further clarification.
Professional Sign Language Interpreting