Yep, camera and locator is the easy way.
Maybe you already mentioned this, do you have a full basement with plumbing downstairs? If so, it's probably around 7'-8' deep where it leaves the house.
If you don't have a basement, it's probably only a few feet deep. In that case, you may be able to probe and find it.
It definitely won't be copper. When was the house built? As the others mentioned, it'll stub out of the house a foot or two in cast iron then transition to clay, orangeburg, asbestos, concrete, etc.
Some municipalities keep really good records and schematics. Others don't keep records at all. If they do have an as-built (schematic); depending on which contractor originally drew them, it could be right on the money, or it may look like a child drew it with a crayon.
When you do find it, I'd suggest putting the backwater valve in outside the house to avoid concrete work. I'd also suggest a backwater valve like this: http://www.rectorseal.com/index.php?site_id=1&product_id=316
. They're easily serviceable, and easy to take out if ever need to snake the line.