How can we know what people want if they hardly know it
The answer is either very depressing or very simple.

The depressing answer is that we can't know it, so we can't organize for
others that they get what they want and we have to leave it largely to
themselves to do so. There are a few things we can do collectively.
Governments that look after the interests of their citizens rather than
after those of the ruling elite usually already do them:
a) Protecting people from malevolent others (among themselves or external to
the collective).
b) Enhancing everyone's options when there seems to be enough
agreement on the general direction in which options should be
enhanced (e.g. mobility, telecommunication, education, health
care etc.).
For the rest we will have to leave it to people themselves to
determine what they want and to get it if they can.

As people usually don't act rationally and just behave in line
with the crowd, we should be quite pessimistic about the quality
they will experience in what they get when left to themselves.
Moving with the crowd gets them stuck in involuntary and constrictive
patterns of behavior that lack meaning.
Avoiding what they don't want usually gets them into situations that
are less fearsome but little more attractive.
Trying to enhance their options in the most obvious way often blocks options
that on hindsight were more attractive. E.g. love, peace, nature etc. can't
be bought with money and are often adversely affected by trying to make as
much money as possible.
Having got what others got before them, people often discover that it has
become far less worth wanting now that everyone has it. Moreover, they
discover that some others have gone on and have got something new which is
much more worth wanting because of its newness.

The simple answer is to just make them want what you want them to want.
How to do that? Create or pass on stories that attach meaning to
what you want them to want.
'Special' things, that can't be easily got, can often more easily be sold as
worth wanting than ordinary things. Apparent ability to organize
satisfaction of wants often makes people more effective in making others
want something.

People who want other people to want something and who organize satisfaction
of that want, let's call them 'political economists', can do so for
different reasons. They can do so to get something for themselves in return
or they can do so to contribute to a better world. These possibilities do
not exclude each other, however. Reasons given may be retrospective
rationalizations of actions that were motivated differently prospectively or
even of (involuntary) behavior.
Even economists who pretend to be apolitical, who only want to
explain how people get what they want and/or to instruct those
that know what they want how to get it, may turn out to be
political economists in disguise. By creating stories explaining
how people get things like 'wealth', 'a higher GNP', 'more
profit' etc. and neglecting to create stories explaining how to
get things like love, peace, nature etc., economists make people
think that the first are more worth wanting than the last.