I generally check with somebody before I hunt anyplace. There are some sites where this is a courtesy because detecting is allowed at them. In my area they include public parks and beaches and schools. I have been asked to leave both a public school and a town park. I had express permission to be at both sites. I still hunt both places today because both officials who asked me to leave did so wrongly. In other words, they had no authority to ask me to leave. I did leave when they asked me to, and then had the town supervisor and school board rule on detecting there and both gave permission. One was a real ugly incident as the cop would not identify himself to me. You can find reference to it about 6 months back on this forum.
Easements in your area might be open to detecting without permission too. Check with your zoning board or local government. Don't be surprised if you find that the citizen who mows the easement thinks it's his property. I try to ask permission or ask if they'd mind.
Private property always requires permission. In some cases this is not easy to get. If the place is administered by a board of directors, it might be difficult to get somebody to make a decision as to whether or not you can detect there. Prepare yourself for a few No's. I find that most folks don't mind and many have even more ideas for great sites.
I thinks the best place to start is at an elementary or middle school. Lots of dropped items. Most in my area are pretty free from trashy items. Fewer pop tops, but more bits of foil from lunch boxes etc..... Do it on the weekend or after school in the late afternoon or early evening. Get to know the custodians and introduce yourself to the principal. Be friendly to the parents and children playing in the area. Respect the grounds and don't leave ANY trace of your detecting. Pick up trash and dispose of it properly.
Post photos of your finds.
Chris