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Quiet rest periods do help. But the best way to tell if an individual's situation is potentially harmful is to get and wear a dosimeter, but they're fairly expensive. You may be able to call around to audiologists or maybe have a friend who works in a factory to see if you could borrow one for a night.
However, there are now smart phone dosimeter apps. I wouldn't use it professionally, but I've used it as a teaching tool. Instruct students to download it and try it out at the clubs etc. I'm an Android guy and I use Noise Dosimeter. I think it's a reasonable start.