https://en.wikipedia.org/wiki/Codeshare_agreement:
A codeshare agreement, also known as codeshare, is a business arrangement, common in the aviation industry, in which two or more airlines publish and market the same flight under their own airline designator and flight number (the “airline flight code”) as part of their published timetable or schedule.
How can I know if a flight number is codeshared, and if so, what other flight codes the flight has?
Since you did not specify at what time you wanted to know this as in before or after buying tickets etc.
One way to know is to look at the departure boards in the airport as they will rotate through all the code share flights for that particular flight.
If you need this information before you buy the ticket it depends on the airline or agent you use but normally they will not publish all the code shares they will simply state their own and the operating airline if that’s is different from their own. They will not specify all other airlines who also code share