Someone on AskReddit asked for people to share the most noticeable, dead giveaways that someone is definitely Australian. If you take a solid look at these and note them for future reference, you might be able to pick an Aussie out of a crowd in the future with no trouble at all. 

Text - AelisWindPiercer 7h Never realised how offensive it was to say cunt until I found friends overseas. I was on a group call over discord and my brother yelled "oi cunt" at me after calling for me twice and getting no response and my friends were appalled, even more so when I yelled back "what cunt?!"