I use the “onX Hunt” app pretty heavily in the field - their mobile phone app is originally intended for game hunters, but it’s perfect for getting property owner information on the fly. I can't count how many times I've used it to pull up owner info when I randomly pass a promising looking location while out driving. There’s a free trial, but then you need to pay an annual subscription ($30 for each state, I think). While the local county tax assessor or auditor’s web site is a primary source for owner info (and much more) for a particular property when I’m researching at home, I’ve found that most county auditor/tax assessor web pages aren't mobile phone or tablet friendly (at least here in NW Ohio) - so onX really shines for field use in comparison.
The app works wonderfully to find property owner names and addresses and the extent of property lines. Also, the app makes it a breeze to determine whether a site is public property or private, which may imply that permission isn’t even needed.
One technique I often use when I spot an interesting “door knock” house while out driving is to peek at the owner info on OnX Hunt and find out the owner’s name, then I have the advantage of using their name when knocking - “Hi, my name’s Ben. I’m a history lover who metal detects as a hobby...I was hoping to find Mr or Mrs Smith for permission to use a metal detector in the yard today...”
Another use for OnX is confirming the extent of a curb strip for those of us interested in hunting those without running afoul of ownership/permission/trespassing issues that have been so hotly debated in some other recent threads. Since OnX definitively outlines property lines, you can easily confirm whether a property stops at the sidewalk or runs all the way up to the curb line - yes, this varies from town to town, and sometimes even from neighborhood to neighborhood within the same town.
I was originally skeptical of paying for the OnX subscription - now I wouldn’t be without it!