Where do they acquire that license? Expert doesn't have license unless they are accountant. But I am sure they have attended seminars about it.

Just try to find some good expert advisor on forums, google etc. And try them 2-3 times on demo accounts. Don't apply them directly on real accounts. And trader should also learn technical properly before using expert adviser. Traders can't depend totally on any expert advisor. If is so, all traders would make lots of money using them.