To be slightly less brutally honest and to-the-point, the George V heads and the 'Roos are an enormously complex area, with some very valuable shades and plate flaws. There are many collectors here in Australia who specialise in them deeply.
As with most earlier definitive issues (and the Canadian Admirals spring to mind) the more knowledge you have of them, the better chance
you have of making the big find, not the person you sell the stamps to. Of course, rare stamps are rare because they're rare: your chances of making the big find are tiny. But who knows ...
If you think you have enough of the George Vs and 'Roos, and you're prepared to make the investment in time and money, you could search for second-hand copies of the Australian Commonwealth Specialists Catalogue volumes on the George Vs and the 'Roos. They'll give you enough detail to be sure you're not leaving any serious money on the table. Beyond them lies hyper-specialist territory, and there's unlikely to be a great deal of extra value there.
Short of that, I'd suggest - again

- get hold of a copy of the Gibbons Commonwealth catalogue. It's far less detailed, but it will alert you to some of the big ticket items.
But I come back to my original point. There's no simple way with these stamps. If you just want to sell them, put them up on
ebay, making sure they'll be visible in Australia, and provide the biggest, clearest, best scans you can. That way you'll maximise your chances of more than one specialist seeing anything of interest that you have.