In the majority of cases, a detached retina has symptoms, but they will depend on the detachment's degree of evolution, which is why they might not be perceived by the patient in the very early phases.
Seeing floaters (moving black dots) and flashes of light on repeated occasions and in the same area of the visual field, may indicate a retinal tear or break, a condition which may be caused by retinal detachment. The appearance of a dark shadow, akin to a black curtain, in any area of visual field is a symptom of an established retinal detachment.
Therefore, if you suffer any of these symptoms, you should see a ophthalmologist immediately so that the diagnosis and treatment—which is always surgery—can be undertaken as early as possible.