In theory, even with antivirus software, you won't know if your machine has malware, as no antivirus software is perfect. AVG has a free Mac Edition you can install. If you really don't want to install anything however, a free online scanner from a reputable firm such as Trend Micro or Bitdefender works well.
In principle though, one can never be 100% certain that their machine is secure. Who knows, maybe tomorrow a scandal will leak out revealing that Apple has hardware backdoors in all Macs. Wouldn't that be something!