You’ll know if an interpreter is competent by looking out for the following traits:
The interpreter uses direct (not reported) speech and idiomatic English
The message is neither excessively longer nor shorter than the original
The client’s responses are meaningful, and there’s no indication of confusion
There’s a smooth flow of information both ways, and the body language matches the message
The interpreter takes notes, asks for clarification or repetition when needed
The interpreter reports any side conversations.