One of the biggest differences between a senior system administrator and a junior system administrator lies in knowledge of their own knowledge.
Both know a vast quantity of information but the senior admin has the ability to say “I don’t know” when he needs to research something further. He does not see that phrase as an admission of weakness but as a promise to not mislead the person asking the question with an inaccurate answer.
A good customer, employer, or colleague will appreciate this trait, especially when the correct answer is supplied in a timely fashion.