The FSS will tell you (bottom L of screen) what % of planetary bodies have been discovered. The majority of inner bubble systems (a sphere about 600LY in diameter) will have few if any not yet found, so the the FSS will report 100% discovered. Move on unless there are high payout worlds as mentioned above. Even if previously discovered and mapped, they are a fair return for your time.
You'll have to go a lot farther out to reach regions where undiscovered systems are common, but they pay a whole lot better! There are bonuses for first discovery of a world, first mapping of a world, more for first discovery of an entire system, and yet more for first mapping of an entire system. So far my largest single system payout was around 15Mcr, but I have seen Cmdrs report more. 3000ly to 5000ly out is where virgin territory usually starts, but a lot depends on how many "tourist traps" like well-known nebulae or oddities like planetary nebulae are in your particular area. The star class of a system primary, the star that's in your face when you arrive out of whichspace, also affects its traffic, as those types that are more likely to sport ELWs, AWs, WWs and other high-end mappables generally get visited more often.