How do I check if a document exists in the Firestore?

You can check it from exists() in DocumentSnapshot.If the Document does not exist, you can also use getData() to return null, so you can decide from that.

If the DocumentSnapshot points to an non-existing document, getData() and its correcting methods will return null. You can always explicitly check for a document's presence by calling exists().

val docRef=db.collection("cities").document("SF")
docRef.get()
        .addOnSuccessListener { document->
            if(! document.exists()) {
                Log.d(TAG, "DocumentSnapshot data:${document.data}")
            }
            ...
        }
        .addOnFailureListener {exception->
            ...
        }
}