You can literally legally detect anywhere if you have permission. Federal and state lands are usually a no-no unless you can get permission. Beaches are usually a safe place for metal detecting, but regulations may restrict you from certain parts of the beach or from going in the water. Parks that allow metal detecting with or without permits expect detectorists to follow local ordinances and to comply with a code of ethics.
Private properties are excellent hunting sites, and all you need is permission from the landowner. If you can prove yourself to be a trustworthy, respectful, and responsible detectorist, you may be allowed back. They may put in a good word for you to friends and other landowners that may have prospective sites for you to hunt.