A basic definition of ‘assertiveness’ is the act of getting one’s needs met but not at the expense of others i.e. a win-win situation.
When it comes to being an assertive speaker, what comes to mind to me is someone that is displaying self-confidence in their speaking. They own the stage i.e. they are comfortable and take advantage of their surroundings. Their speaking style uses active language rather than passive or subservient.
Being assertive is not being afraid to talk about your subject and expressing your personal opinions. Many people are hesitant or afraid to express an opinion publicly. A big part of having an opinion and expressing it is being able to defend your opinion, both logically and emotionally. This can take a good level of assertiveness to do so.
One characteristic that I see in a non-assertive speaker is the raising of the last syllable in a sentence. A statement comes across as a question when it isn’t one. It affects the speaker’s credibility. The speaker seems unsure of themselves. If you have this habit, a good speech evaluator or a speech coach can help you become aware of the habit and extinguish it.
Becoming an assertive speaker takes practice, lots of it. It not only takes practice, it takes constructive feedback to help you improve.
Toastmasters clubs are definitely a great place to work on developing your assertiveness in speaking.
Thanks for your question.