Check the mass-code of the star system; it's the last letter in the system name, the one by itself before the numbers start. For example, in the Thraikoo BR-M c7-21 system, the mass code is "C". Mass codes range from A to H and, as a general rule, the larger and more interesting stars occur in the later mass codes. You will find, as you travel the galaxy, that all the mass code H systems and most of the mass code G systems have already been explored, because there are explorers that systematically survey the galaxy by mass code and seek out those high mass code stars.
There are also "tourist spots" around the galaxy - most notably, the nebulae - which tend to attract more visitors then the less interesting parts of the galaxy. You can be guaranteed that every star inside a nebula and most stars within about 20 LY of the larger nebulae are also already explored by now.
I systematically surveyed the Thraikoo sector, over 1400 stars of all different types and mass codes. The only Tags I ever saw were in mass code G and H systems, and in the systems surrounding the only nebula in the Thraikoo sector. The rest of the sector was entirely Untagged.
Obviously, it's also possible to land on a perfectly ordinary star system that's already been Tagged. Do something often enough, and even extremely improbable things are bound to happen. The probability of this happening depends on the density of stars in the area. Fewer stars mean it's more likely that anyone passing through that area will have visited those particular stars.
I'm currently surveying a Core sector. The direct-line route between Colonia and Sag A passes through this sector. I passed through this sector myself a couple of years ago flying from Sag A to Colonia. It's a very popular route. Yet even here, almost all the Tags I'm seeing are in G and H systems, and in systems around the nebulae. I've surveyed 800 systems in this sector so far and I've only seen one "random" system previously Tagged by someone else. Why is it so? There must be tens of thousands of systems in this sector that are already Tagged - but there must be about a billion systems in the sector overall - so Untagged systems still greatly outnumber Tagged ones and finding a Tagged one at random is still very low odds.
