If the owner neglects his trees, leaves the fruit to fall off the trees and rot, and doesn’t care at all who picks fruit from his trees, then the fruit is considered hefker (ownerless), and you are not required to separate terumos and ma’asros from it (assuming you pick a normal amount).
If the owner tends to his trees, yet he gives permission to everyone to pick some fruit, it is doubtful whether the fruit is hefker or not, because the owner has not explicitly made it hefker. In such a case, we are unsure whether the fruit is considered hefker or a gift, and terumos and ma’asros should be separated without a brachah.
Fruit growing on trees in public places is considered hefker if the local authorities do not pick the fruit or give the rights to the fruit to a specific person. Terumos and ma’asros are not separated from such fruit.