From my experience cutting, splitting, and burning wood for a wood stove, you should not have too much difficulty telling hardwoods from softwoods. Hardwoods should tend to keep a darker color also. Chances are if the tree is white or gray looking, it's a softwood. These are all excellent points. Also, now that I think of it most of the big hedge trees I've cut the past few years tend also to bend or fork over creek beds they are/were on.
We follow treelines on lakes when we go for catfish because they help you find the channels. The bigger the tree, the closer to the channel it probably was. Also remember though that these are rules of thumb. In many lakes where the channels are huge and the channel drop-off can be 20-30 feet deep a lot of these trees you can use as indicators have actually wound up laying at the bottom of the channel. Especially in old lakes. I think fishfordollars mentions of drains would help more in this case, but I'm not quite sure what he means by a drain.