Is there a way to check dental decay in people living with/consuming hard water?

Yes: Dental decay must be detected by visiting your dentist and taking the proper x-rays. It is very difficult to detect dental decay on your own.

Yes: There have been studies linking some of the minerals in ingested water with decay. Specifically flouride. Hard water by defenition means that the water has high mineral content. It would be more interesting and relavant to test each mineral as it pertains to decay vs "hard water" that varies from well to well, city to city.

Absolutely: By visiting your dentist. Your dentist is trained to detect decay, and with the proper radiographs, s/he can see between your teeth where the decay would otherwise go undetected.

Dental cavities: Yes, see a dentist for an exam. I have not ever seen a study linking hard water to tooth decay. If your water is lacking fluoride you could be at increased risk to dental decay. Go see a dentist for an exam and they can make sure your teeth are ship shape.

Dentist: An exam by your dentist will detect any dental decay.