Answer to that question, is most you will already know. Most parks, playgrounds, nearly all beaches have been around for years. Use your melway sydway or what street dirrectory your town has in the rear is a list of parks. The age of the town is usaully a good indication for age of park as they don't often move. That would be where I start for old coins. Most copper coins arn't worth anything as they corrode over time silver coins don't in most cases. My first Threepence was picked up on a beach I rubbed it to see the date and it snapped into 3 pieces it was so thing from salt & corrosion.
My research for railways, old building sites, private property etc. comes from many books, internet, word of mouth. When searching you may find ten or twenty old places such as railway stations but then out of those a lot of them will be on private land now. You might be lucky the ones you search might all be on Crown land, but they might be all on private property.
So then I use
www.dpi.vic.gov.au & geovic to see if it is crown land. Maybe that's a link that one of the moderator's could add.
That is a Victorian link the other states may also have something. Even this site is complicated if you arn't familar with it.
When searching like this it does get difficult but you can find some good sites. My suggestion is if you are unsure or starting as a new hobby, join your near metal detecting club You can learn heaps from just being with others doing the same hobby.
If it is not crown land you need permission.
Some crown land like Historic sites, Fauna parks, National Parks are no go zones. If you go to somewhere like Castlemaine in Victoria ask the Tourist Information centre where you are aloud to go. Some Caravan Park owner like Laancoorie, Wedderburn, and others can also help. These places can also help put you on the goldfields also.
Hope this helps.