By "power line" do you mean a pylon on a high-power main electric line?
One thing you could do is buy or borrow a gaussmeter and just check out the readings in the place where you would live. I did this the last time we were buying a house. I got a $25 gaussmeter and discovered that our new house was fine... but our old house wasn't! We weren't even by a really high-power line, just one of the bigger lines that was carrying power to a lot of the neighbourhood.
A hint, should you decide to do that, is to make sure there's no heavy power use in the house at the time you check it out. Lights off, etc. You'll probably find there are at least some "hot spots", probably close to where the electric line enters the house, but you can get a feel for the house as a whole and especially the bedrooms.
I've been worrying a little that my ds's new school is about 0.5 mile from a substation, but the school is really the best choice for all kinds of reasons and I figure home is more important since he's there more.