Ask her to talk, not to score herself
Open with something she cannot have memorized as a fixed script: what her typical day looked like in her last household, how she'd handle a specific situation you describe, or what she would say to a child who won't get ready for school. Listen for whether she is constructing sentences in real time or reciting a prepared answer, and follow up on something she just said rather than moving straight to your next question.
A single unscripted follow-up question does more than the entire rest of the interview to show you whether she is actually following the conversation or pattern-matching to expected questions. If her answer to the follow-up doesn't connect to what she just said, treat that as more informative than a fluent-sounding opening line.
- Ask her to describe a routine, not to state a skill level.
- Ask one unscripted follow-up question tied to what she just said.
- Give an instruction and ask her to repeat it back in her own words.
Set the bar at what your household actually needs
Decide first what English is actually for in your household: following instructions and reporting problems, or something closer to holding a conversation with a child, tutoring, or managing a detailed medical or dietary routine. The level worth testing for and the level worth expecting are different depending on which of these applies, and it's worth being explicit with yourself about which one matters before the interview rather than after.
Most candidates land somewhere in functional territory: clear on everyday instructions, shopping, schedules, and simple exchanges, less confident holding an extended conversation on an unfamiliar subject. Treat that as the realistic middle ground, not as a shortfall to be corrected quickly, and build in a short settling-in period where you both adjust to how the other communicates.
What this assessment can't tell you
An interview conducted in a formal, slightly nervous setting will usually understate how comfortably someone communicates once she's settled into a household and picked up its specific vocabulary and routines. Treat a shaky interview performance as one data point, not a final verdict, especially if her comprehension of instructions was solid even when her spoken sentences were short.
If communication is one of the two or three things your household cannot afford to get wrong, for example close daily coordination on a child's schedule, say so plainly in the interview and weigh this alongside her other experience, not instead of it.