if you were to go for an normal eyetest check up would they be able to tell just from that if you were diabetic?

They could probably tell if the blood vessels/nerves in your eyes were damaged, but diabetes might not be the only cause of that.

Considering it's dangerously high blood sugar that causes that damage, you'd probably have noticed before - what with the throwing up and the death and all that.

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The normal reason they ask if you have a family history of diabetes at an eye test is to look for glaucoma (which can be linked). Retinal damage would be a sign of very severe uncontrolled diabetes, so generally you'd know before that. If you are worried about diabetes, go to your doctor and get it checked out.