I find that the majority of edge cases come from the business system (the business has a system/process that we turn into a technical system). I try to approach it like a lawyer trying to find a loophole. Try to ask questions that challenge assumptions about what conditions are possible in the system and ask questions that seem absurd but fit the requirements.

For example, if you're building a financial system maybe ask if a negative balance is allowed. If you charge a fee based on balance (like using basis points), can the fee be negative or is there a set floor? A negative fee sounds stupid, which is why the business probably wouldn't mention it in requirements, but it would be important to set the floor at $0 so if a negative balance occurred, we wouldn't issue a fee.