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!
Outside of very obvious activity; unexpected Pop Up`s, Ad`s etc. no you need a software solution. For OS X Malwarebytes or Bitdefender (free version from Ap Store) are as good as anything else out there and both are run on demand no daemons or start up Apps.
I like Malwarebytes myself, and recommend that. There's really no way to determine if your machine is infected with malware without some software scanning the system and looking to detect the telltale signs of infection.