If you can find a stream with no human settlement or factories along its banks it "should" be safe to eat.
Cooking the fish really well should kill most bacteria but heavy metals and toxic chemicals wont be removed through cooking.
Me personally wont eat a freshwater fish as all our rivers and dams are full of sewage never mind all the other toxins dumped by factories.
So in short a clear berg stream that has a trout or 2 I guess you can try your luck ,but the bigger dams/ rivers I will stay away from.
That said ALLOT of people eat fish out of all the dams and rivers and they are still alive. So it comes down to is it worth the risk?
Is there a easy way of telling what you can or cant eat? No.