Hey you can use methods like stress testing to find the test case you are failing. But yeah codeforces should give you the testcase you are failing.

Also I think that lets say even when you get the testcase you are failing it wont be of much use to u if it is large testcase as you wont be able to run it down on a notebook to figure out what went wrong.

Instead make generators with small constraints to at least check if you're approach is correct. Then you may proceed to test on larger inputs.

Errichto Video

similar video in Hindi

I also face the same problem. Sometime I can see error in a particular test case but the test case is too large to see. There should be more flexible options so that we can debug more efficiently.

Codeforces may also use systems like LightOJ so that we can compare the result the whole test case results with an accepted one.

Checkout CF Stress

if the case is small and not very big you can always print it just when printing it don't print any spaces at all