How do dentists tell if you have enamel loss? do they always check for enamel loss?

Visually: The answer is, yes, dentists do check for enamel loss and it is determined by mainly visually inspecting the teeth during an examination. In cases of severe enamel erosion, the underlying dentin layer can be seen indicating an increased chance of sensitivity or decay. In severe cases, it can also be seen on radiographs.

Use magnification!: Most dentists wear loupes to magnify our findings. A microscope camera is another tool we use to find damages at its earlier stage than obvious to the naked eyes. I see you are only 18 y.o. and likely not have much enamel loss, but find a dentist with loupes to find damages early enough, so you learn about the reason(s), and what options you may have. Without enamel, teeth are damaged too fast.