Even we cannot use an EEG (Electroencephalogram), we still get a very good picture of your REM phases from your activity with a high match rate.
You can read about how we do it in https://sleep.urbandroid.org/how-do-we-measure-your-dreams/

Lucid dreaming is a technique which allows to become aware of the fact that you are dreaming in order to gain control over your dreams.
There are various techniques how to help your mind realize you are dreaming and most of them require some training in advance. The technique we use in Sleep as Android uses audible cues played (ideally in your headphones) during sleep. We play the cues usually towards the end of the deep sleep phases where we expect the most memorable dreaming. Hearing the familiar audible cues should help you realize that you are dreaming. Please refer to other resources online on how to bed train to induce lucid dreaming and how to train yourself on the audible cues.
Richard Feynman’s Surely You’re Joking, Mr. Feynman! contains a very nice description of a set of lucid dreaming experiments done by this famous physicist.

Settings → Sleep tracking → Lucid dreaming
Settings → Services → Smart light → Lucid dreaming