The idea of 100% faultless virus detection is a classic among malware researchers and hobbyists and as far as I know, there is no really 100% guaranteed way to tell that a file is not infected. Even if you download the source code, examine it thoroughly, and compile it yourself, there's an infinitesimally small chance that your compiler itself is infected and infects every compiled application. And in modern times, there's absolutely no way to scrutinize the source of every single program you're using. That's why it's necessary to put some trust into your antimalware software, and besides, most of it is really good at detecting malicious code. If you don't find results from one or two packages reassuring enough, use a website like virustotal.com.
There's also the fact that many people already use those tools and there's quite a few advanced computer users among them -- the kind most likely to notice when there's something wrong. If there are no reports of infected files, AND if they come up clean when scanned by antivirus software, then they're most likely safe.
