(a how-to-guide for International readers)
1. Unusual accent - occasionally mistaken for British or Canadian until flatter vowels are noticed or okkerisms are applied (Struth! Bonza! Bewdy - Havagudweekend).
2. Unique sense of Humour - we do humour well and often - sometimes on the crass level. We are also noted for applying even more humour when the situation is grim. How can you tell that the situation has hit Ground Zero? The Australians in your midst stop the jokes and start saying "She'll be right"
3. Liberal sense of self - the "average" Aussie (ie. not a wowser*) won't usually get uptight about religion or politics. Sure we can discuss both over a barbie and a beer but we have a certain "your business is YOUR business" attitude and allow everyone their own beliefs even if we (privately) think you are a raving loonie for having them (unless of course you insist on voicing said beliefs loudly and aggressively after which you'll have your beer cut off and shown the door)
4. How do you tell an Aussie service person? - check here for the various shoulder patches. Notice we don't fly our flag in the reverse (I'm still waiting on an easy to understand explanation of why the USA does that - A1S promised to explain months ago but then he got famous. sigh...)