Walk the terrace and look for standing water marks from last year. Water that sits is water that will find a crack. Slope correction with a screed is a two-day job in winter and a nightmare in the rain.
Check the parapet junction. Most terrace leaks start where the slab meets the parapet wall, not in the middle of the slab.
Look at the external wall on the weather side — in NCR that is usually the west and south. Hairline cracks in plaster take in water and show up as a damp patch inside.
Check the chajja and window sill drip moulding. A missing drip groove sends water straight down the wall face and into the window frame.
Open the bathroom sunken area if you have access, or watch the ceiling of the room below for a shadow. Sunken slab leaks are slow and are usually noticed a year late.
Clear the rainwater outlets. Half the terrace leaks we see are simply a blocked drain that turned the terrace into a shallow tank.