I think you need to understand your character's reasons for being a Know-It-All. It can be out of enthusiasm for a subject or deep need for acknowledgment or a need to show up other people or even an extreme fragility of character. Each of those manifests Know-It-Allism for different reasons.
Enthusiasm -- is a geeky know-it-all. Overshares. Tone can be excited, lots of details and facts and cool contradiction. This is like Hermione Granger from Harry Potter, esp first few books.
Deep need for acknowledgment -- for whatever reason (too many possible to enumerate) individual put great effort into learning a subject but feeling inadequate, constantly seeks positive reinforcement by blurting information, needing people to say 'good job' or trying to recreate a feeling of accomplishment by having others react to their mastery of the subject. The tone is not excited as in Enthusiasm but might be nonchalant and casual or focused. Determined by specific need character is trying to satisfy by being a font of knowledge. This is most any person answer questions on Quora and Stackexchange.
Show-offs and Fragile egos -- both can come across as very smug and pretentious, treating their subject matter with great reverence, no matter how difficult it was to master. The tone is slow and deliberative because it puts them in control of the conversation, draws out everything, overly conscious of giving away to much knowledge. Will share enough to show they know what is what, but a reticent to share what they know for fear of diminishing their own value. They make also be trying to puff themselves up by appearing more authoritative then they really are. Unfortunately, this is most likely me.