One doesn't require an expert to tell one what one already knows.

(humorous) It is easy to conceal one's identity on the Internet.

(informal) very quickly

One can never be sure of one's paternity.

Third-person singular simple present indicative form of know which side one's bread is buttered on

Synonym of you never miss the water till the well runs dry

Third-person singular simple present indicative form of know the difference between one's ass and a hole in the ground

(Commonwealth of Nations) New management imposes radical changes, but those with prior experience have more knowledge.

Identified as a specific type; famous, renowned. / Accepted, familiar, researched.

Any fact or situation which is known or familiar. / (algebra) A constant or variable the value of which is already determined.