Most exchanges which offer fiat-crypto pairs implement KYC as a legal requirement. This is usually done by verifying identity documents.
Most exchanges require a registration process which range from near anonymity to full identity verification (KYC - Know Your Customer). Some exchanges offer trading pairs between cryptocurrency assets exclusively, while others offer cryptocurrency to fiat pairings.
In general, the closer an exchange deals with fiat, the stronger its KYC requirements are.
* All terms and definitions may change as the Cryptionary improves