Expertise cannot come by vanity. You should be able to react and respond to what other people already think about a subject. Your expertise has to continuously look for variety.
A true expert will never fail to enrich her or his experience in the niche that she/he is already an expert in. He will always look for ways to improve what he already has. In the beginning, knowledge can be gotten using educational material and books; afterwards knowledge will come from experiences. This is the way that someone learns something at college and takes it up as his career. When you are at school, the knowledge you gain comes from books, when you begin to work on your job in the field, your knowledge is about the experience you get on the job. This can be done using inference and observation.
If you want to become an expert in the niche you choose, you should always keep this in mind – you always have to look out for what other experts in the niche are doing.
Think in this way, a person who asks you a question as an expert may go to see someone else. If the other person is able to say something that is more current or more substantive than you then you will no longer be considered as an expert by that person. This is why you must always look for what is new in your niche so that you always have up to date knowledge.
Observation can be used anywhere, you can watch television shows that talk about your area of expertise. You can attend a course or a seminar to get a chance of seeing another expert. Another source of observation can be YouTube since it has different videos on different subjects. You can make watching these videos one of your hobbies and you will be gaining the updated opinion about your niche as well. Keep in mind that this can be used as a precursor to claim your position as an expert. You will have to take a long way. You have to enrich your experience and your knowledge so that you can begin to think about calling yourself an expert.