Hydrocephalus is best detected antenately by:
X-ray abdomen
Amniocentesis
Clinical examination
Ultrasonography
Hydrocephalus is a condition in which there is an abnormal increase in cerebrospinal fluid within the ventricular and subarachnoid spaces of brain.
“The prenatal diagnosis of hydrocephalus is usually made by demonstration of a dilated ventricular system in an ultrasound examination.”
Fernando Arias 2/e, p 331

Earliest and most accurate sonographic sign of hydrocephalus – Enlarged lateral ventricles.
The lateral ventricle is measured at the level of atrium.
Normal transverse diameter of atrium is 7 mm + 1 mm (It remains constant during the second and third trimester).
When diameter of atrium is >10 mm, it is called as Ventriculomegaly/Hydrocephalus.

Other signs of hydrocephalus on USG:

Dangling choroid plexuses.
Thinning out of cerebral cortex.