That is a great question with a multi-part answer: I have close to 50 different landing areas in about an hour radius of where I am based. I have cultivated those spots over 12-15 years. First you have controlled/public airports - self explanatory. Second you have RLA's, generally someone is listed as an owner/manager in Foreflight, There is a phone and an address. Generally I write to the address so I don't bother them with a call. I give them my email, cell, address and details of what I want, what type of plane, a picture. I generally tell them one landing, no touch a goes, not too early - etc. And then you wait for a response, most of the time there is a connection somehow to EAA, someone knows someone, you run into them somewhere. About 5 of the places I land are ex patients or their grand parents own the strip ( you may not have that connection opportunity ). Of this group, RLA group, I have only had two solid no's - don't go there don't even fly over low. Sometimes there is not a hard yes but a wink and a nod, go to these occasionally. If someone gives you a hard yes, there is no plausible deniability - they gave you permission to land outright - these types, the wink and nodders, are fine with you landing, but if anything happens they can say I never said he could land. Then there are the unlisted open fields that you know look like they own an ultralight, for this there is an app called onX Hunt. I use it for flying and hunting, it will list the land owner with an address - same letter scenario. I once had someone wave at me as I was circling flying over, a wave is permission in my book, landed talked and is one of my go to places now. Cropduster fields are awesome, the land owner leases to a crop-duster service, if you contact the landowner they will tell you who leases it, or you find out through the aviation grapevine. Generally if you avoid crop-duster fields during spray season they are fine, indeed appreciative as you can email them with problems with the runway - coyote holes, gopher holes etc. Be aware, sometimes these fields are only cut once or twice a year and are used for hay also. A low pass and drag is a good idea - see attached video. Lastly there is the just go for it. In my time in the Air Force, 1983 - 91 era, we had a saying "Better to ask forgiveness than permission" This carries it's own problems which I cannot recommend