Sending checks through the mail is generally secure as many people frequently mail checks safely to payees. Use reputed mail carriers such as UPS, FedEx or even the US Post Office, but ensure that you track the check.
It is essential to avoid sending checks that are payable in cash as they can be easily cashed if they land in the hands of the wrong people. Write that the check is payable only to the payee's account in the endorsement area on the back of the check. Other secure means of sending money include wire transfer by companies such as Western Union, payment services like PayPal or Google Wallet and through bank transfers.