Originally Posted by kazak4x4
Youtube has a ton of videos of all the popular runs and rapids.
What he said. I have "scouted" quite a few rapids and rivers using YouTube. Keep in mind that the flows on any given day can dramatically change a rapid, so take note of the flows if they are included in the description and watch several at different flows to get a feel for how it changes.
I also will go for family car rides and hikes along those that are nearby and that have easy access from a road or trail. It's fun for the kids (what kid doesn't like throwing rock in rivers? LOL) and I can take a look at features of the river.
If in doubt, I'd say be cautious and scout anything you may be unsure of, even if it's supposedly a class III for example. Give yourself some confidence going into whatever it is by scouting and having a solid line picked out.
The BEST way though is to get out on the river with others that know them and follow them. Try to get in the habit of making your own read of the river, but at least you can test that read against what those with the experience do.