As far as the forest, if you can find a big enough clearing where the plants will at least get a few hours of sunlight at mid-day you at least should get some sort of harvest. One way to estimate the hours of light is to hold your hand, fingers down, out at arms length toward the clearing. Each finger is 15 minutes, and the sun will move "four fingers" in an hour. So count the number of "hands" across the clearing taking the same path the sun would take (generally east to west). Then you can see how many hours you'll get each day. Not all plants need direct sunlight, but marijuana does.

Another way is to look for clearings that are big enough is to look to see if wild grass grows there naturally. Marijuana has very similar sunlight needs to wild grass, ironically.

I would stay away from the power lines or anything man made. You want to find an area humans would actually avoid, not just a place humans hopefully will not not wander by. The chances of humans wandering by power lines is pretty good.

The place I found that is perfect is in swampy area way back in some undeveloped land on the outskirts of my city. It is nasty as hell back there. Last year I set the plants back in there next to a briar patch with some other bushes around all sides so that someone would have to walk right up on the plants to see them. And no one in his right mind would be walking through that swampy area anyway. In the early spring it wasn't bad, but by summer the place had 6-7' tall grass, snakes, ticks, etc. I had to cover my whole body and tuck my pants into my socks just to not get bit up (learned that the hard way). I also sprayed a high concentration DEET ("deep woods OFF") bug repellent all over my clothes before I went out. And as soon as I got home I had to throw the clothes in the washer, shower, and check myself for ticks. I got some chigger (mite) bites even with all that protection. But it was worth it to make as sure as possible I wouldn't end up in prison.

That is the kind of place I'm talking about. If you can find a big enough clearing in the woods, that could work. But I recommend a nasty, "hard" area as opposed to just anywhere in the woods any hiker might stumble upon.