I found one, after noticing some things that help.
1. Look for a small planet, 500km is a good size.
2. Look for obvious signs of erosion that stand out on the planet map, like a deep valley or what looks like an empty lake bed.
3. When you've dropped out of supercruise above the planet, you'll see your heading in degrees (0-360). 0=N, 90=E, 180=S, 270=W. I used a topographic compass to make it a little easier, but you can make one on a piece of paper easily.
4. Once you figure out your heading toward the area you want to check, supercruise to a few spots that stand out on the planet map, and drop out of supercruise, staying at least 3.5km above the surface (if you're too low, the radar blips won't show up).
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If it takes too long, or the planet is packed full of crags and valleys, just move on and don't spend too much time.