What is the best technique to detect your client needs?

Very important question, one method I use to really know what the client needs is to start adding constraints on what they want.
The moment the client see that they cannot get everything they want they then start to prioritize.
Once they start prioritizing you will notice that there are somethings they want more than others.
Then you can question them about those high-priority wants if they can really not live (or continue business without it).
In short, one method is to add constraints in order to cause the client to prioritize their wants, and the high priority wants will end up being their needs.
There are other methods, but this is the simplest one I could think of right now.