Yes, there are various icons when you hover over a star, one is the system map. If this is red then you haven't been to that system.
I'd also say the basic Disco (discovery) scanner has very limited range but when you arrive at a system if you look in the left menu it should still list stations and so on. If you scan when you arrive and the system map still doesn't show the stations and planets you can target a station (from the left menu), fly out to it and scan again when closer.
It's not a vast amount of credits though, some systems you can pick up just a few 100 Cr in others it can be 20,000 Cr + for the data and you have to be 20 LY away from that system before you can sell it. Still, early game play, it can make a big difference if you're not really into combat and just want to explore.
Oh and it's not unknown systems as in ones no one has explored but just new to you, to get really new systems be maybe 1,000 to 1,500 Ly away from the bubble, but get at least a Hauler with an A rated FSD for those kind of journeys (although people have been to the other side of the galaxy in a sidewinder!).
