You don't speak your client's language - that's why you've hired an interpreter. How can you be sure that what they're conveying is, in fact, the full and accurate message? You're aware that the interpreter probably doesn't have a formal qualification in your line of work, so are they passing on the information correctly?
There are some useful signs to watch out for that will give you an idea. First of all, check the interpreter's behaviour. If they're competent, they will be using direct speech (not third person reporting) and idiomatic English. The message in the target language won't be extraordinarily longer or shorter than in the source language. You'd also expect to see the interpreter taking notes - dates, numbers, lengthy statements - and asking for repetition or clarification if they missed something or didn't understand it. Additionally, they will be reporting to you any side conversations they may have had with the non-English speaker.
Observing the client also provides some clues. Is his/her response relevant and meaningful? (Take care - there are occasions when this won't apply, for example in mental health settings, or when a client may be trying deliberately to confuse.) Is there a smooth flow of information in both directions? Additionally, check to see that there are no indications of puzzlement or confusion (see previous warning), and that the emotion or body language matches the words in the message.
You also have a leading role in this - it's your conversation. Ask the client (via the interpreter, of course), to repeat back some of the instructions you've given him/her, just as you would if you were checking the understanding of an English speaker. Another alternative is to ask them specific questions that will tell you if they got the message or not. For example, "How many times a day do you need to take this medication?", or "How much of this product should you use at a time?"
Don't be tempted to use the (closed) question "Do you understand?" The most common response is to please you with a nod or smile or "yes", regardless of the truth - but it most certainly won't confirm real understanding.
If at any time you think the interpreter simply isn't competent, stop using them. Signs that they aren't up to scratch are such things as giving their own opinion, taking sides or taking control, or having unreported side conversations with the client. Messages in the target language significantly longer or shorter than those of the source language are another indication.
You'll find lots more detailed guidelines on working with interpreters elsewhere on this Qi site, as well as the indicators of competent versus incompetent interpreter behaviour.