How do I implement server-side validation with socket.io?

Consider this basic socket.io code.

Client:

function chat(message) {
    socket.emit('c2s', message);
}

socket.on('s2c', (message) => {
    console.log(message);
});

Server:

socket.on('c2s', (message) => {
    io.emit('s2c', message);
});

There is no form of validation here, and there is nothing stopping the client from sending data to the server via inspect element. How do I go about validating the incomming data on the server side, such that anything sent manually by the user is highly unlikely to be valid?