Sometimes fuses will look good; but will be electrically open, so it is risky to assume fuses are good just by their appearance. Better to replace a suspected fuse with a known good one. If the outlets were originally on the car; the circuits which feed them should be listed in the owners manual, and the fuse for that circuit should be identified as "accessory" or something similar on the chart on the fuse box cover. If the outlets were added on to the car after it was purchased; they might not be fused; or could be connected to any circuit on the car.
But problems with the outlet could just as likely be a bad ground connection for the outlet as a bad fuse. So you need to locate the ground connections and clean all the terminals and contact points.