Back to Q&A board 
You do not need to predict every hidden test. You need a repeatable way to find the assumptions your solution depends on and then try to break them.
I use a checklist called BAD SIGNS:
B — Boundaries: First and last index, exact thresholds, inclusive versus exclusive ranges.
A — Absent or empty: Empty input, missing target, or no valid answer.
D — Duplicates: Repeated values, ties, or an input where every value is the same.
S — Single or small: One element and the smallest nontrivial case.
I — Input extremes: Zero, negatives, large integers, or unusual values allowed by the problem.
G — Giant input: Will the solution meet the time, memory, and recursion limits?
N — Natural ordering extremes: Sorted, reverse sorted, or all equal inputs.
S — Special structure or state: Cycles, disconnected components, overlaps, or source equal to destination.
You do not need a test for every letter on every problem. First clarify what inputs are valid, then choose one small, high-risk example for each relevant category.
The most useful step is to state an invariant: what must remain true every time your loop runs? For a sliding window with no repeated characters, for example, the window must remain duplicate-free and its left boundary must never move backward. Testing "abba" exposes implementations that violate that rule.
Before submitting, do a quick line-of-code attack. For each risky operation, ask: Can this index go out of bounds? Can this map lookup miss? Can this stack be empty? Can this arithmetic overflow? Then dry-run the smallest valid input and one adversarial input, and check worst-case complexity.
That is much more efficient than trying random examples until the solution “looks right.” The goal is to turn corner-case testing into a short, consistent part of how you solve every problem.
I explain the method with worked coding examples in my BAD SIGNS code robustness guide.
Happy coding!