I’m a bit of curious about this because about half of my ECs have something to do with either international or national competitions. Not to mention that I’ve participated a quite well and received valuable certifications. But the thing is do I really have to corroborate? or do college admissions require proof of any kind of ECs?
No they do not, integrity is expected, but international and national compeition winners are usually on the internet and some lists are distributed to colleges. And it would be strange if someone did something extensively or multiple things and those were not mentioned in the GC letter, or your essays.