Detecting water molecules is like knowing a person. A static picture of a circle of friends can't give us much information. In order to understand deeply, we must "observe what they say and what they do".
Fortunately, water molecules are masters of square dance: they are constantly moving all the time, which is called molecular vibration. Each molecule vibrates with different energy. We can use spectroscopy to detect the frequencies of various vibrations, just as our ears listen to sounds of different frequencies.
How to distinguish water molecules
However, so far, we haven't mastered the technology of adding water molecules around a water molecule accurately, and the molecular spectral signal of a water molecule is too weak to be detected at all. Humans can't do it, but fortunately computers can. By establishing a model by computer, we can simulate how the spectrum of W changes when water molecules are added around W.
There are two main directions of molecular simulation in chemistry. One direction is to use quantum mechanics to simulate the quantum interaction of every molecule in the system, including every atom and electron in the molecule, which has a huge amount of calculation and is mainly used to study the static characteristics of molecules.
The other direction is to use molecular dynamics method to imagine molecules as rigid atoms connected by springs, and the interaction between molecules mainly considers electrostatic interaction, so the calculation amount is small and the dynamic process of molecular vibration can be simulated conveniently.