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Verified probably nowhere
As said, no where.
There is no verification process and anyone can claim to be a “developer”.
What you can do though, is check their forum account here, to see how long they have been on these forum, ask to see their past projects or GitHub/similar accounts to check past works yourself.
And definitely stay away from websites live Fiverr or anyone that has the words “Best”, “Most optimized” or such in their posts. As there are billions of ways to create the same result, so nothing can ever truely be the best way to do anything.
These are buzz words used by con artists and people still take their bait… Makes no sense to me.
Basically use logic and verify them yourself.