The credibility of a speaker is established by a host of elements that have to do both with the mode and delivery of the speech as well as non-verbal cues and body language. Some of these factors include:
1.) Tone of the speaker- is the speaker coming across as confident and assured?
2.) Physical presence- does the speaker have a firm and upright posture?
3.) Message delivery- is the speaker presenting the topic with appropriate grammar and language that communicates the message effectively to the audience?
4.) Knowledge of subject being presented
In some cases a speaker may have credibility or establish credibility just by being a known subject matter expert. An example of this would be Jack Welch former Chairman and CEO of GE giving a speech on business management and growth. He has an established track record and proven authority that establishes credibility with a given audience even before going into the speech.