Look up Cariboo Outback Saddles and click on the link "horse saddle fitting" on the right side of the main page. They have provided a good guide with visuals of how to measure and fit an Aussie saddle. They also sell girths, saddle pads, Aussie bridles, etc.
Chances are that if your saddle doesn't have any clear markings, it is a relatively inexpensive Australian saddle, one of the "knock off" brands. This doesn't mean it isn't a decent saddle, just that it probably didn't cost a fortune new and probably wasn't made in Australia. The Australian made, genuine, nice quality saddles will start at about $3000 or more USD. The leather will be supple and the stitching and flocking of a very high quality. The hardware and fittings are also much higher quality than the cheaper saddles. The "no name" brands can be purchased for as little as $250 and up. While the leather isn't as supple and the quality not as good, a well oiled, older saddle will still work very well and be quite comfortable and very serviceable.