In the left hand panel that lists all the system bodies and systems within jump range it will list them as High/Low Intensity Resource Extractions Site iirc, they will only show when you are fairly close to them, I can't remember what the specific range is. As Surfer said, look (open the system map) for rings systems around planets, metal/metal rich ones are the ones with extraction sites, ice rings will not have them, unsure about rocky ones.