Address Public Private key verification
No, there is no enforcement that a private key exists. 

However, due to a Bitcoin address including a checksum, it is very unlikely for the client to create a transaction when for an address with a typo, paste error, or other erroneous data.  There are addresses that will pass the checksum test in which, presumably, no private key exists. 1BitcoinEaterAddressDontSendf59kuE is, in all likelihood, one such address.