If you don't want a false result, then testing the day that AF is due is always safest. However, it you want to test earlier then it depends on the test that you use. I have always found that First Respose works really well and you can get them from Woolworths. You need to buy the ones that are for 5 - 7 days before AF is due and test around 4 days after you think implantation happened - you may get a faint BFP.

There is heaps of info on HPT on this website - peeonastick.com - it should have info about the different brands of HPT and when they will show a positive result.