Most companies have a financial calendar. In many cases it is on their web site. For example, I went to www.asx.com.au, went to company information for Hills Industries (picked at random) and clicked on the link to their web site. Clicked on Corporate Information and then on Financial calendar and there were the dates.
If you cannot find the dates on their web site, you can contact the company and ask if the date has been determined.
Some broker web sites enable you to set up a list so that you are alerted if there is any news on a company. This might be useful if you only know an approximate date.
Sometimes the media will have done some of this work for you. The Australian Financial Review often indicates the expected results announcement for the next week for major companies. However, they do not generally cover smaller companies.