If you're in an area but not specifically aware of any rock art, just knowing where you're likely to find it will help narrow things down a bit. Rock art seems to mostly be on south-facing walls, but that's not always the case. Looking in alcoves, under overhangs, etc., is also usually a good bet. I'm always scanning canyon walls as I hike, and I often stop just to concentrate on the likely spots for a few seconds.
I've also had some really good luck from sleuthing online. Once I find a photo of some rock art I want to find, I can usually match up some details (landmarks, type of rock, even individual trees) in the photo with the terrain in Google Earth to get a really good idea of where it's at. I've probably found a dozen rock art panels by doing this. Also, if the photo is on Flickr, clicking on the link for the date the photo was taken will show you all the other photos that person took on the same date. Using the timestamps on those photos, you can sometimes use the times to judge distances between landmarks and the rock art. Googling the name of the panel will usually give you photos from other people to help narrow things down a lot.
