Google is your friend, as are the numerous online modeling websites.
Let me explain. Just about any kit out there has already been reviewed by a website or magazine. Some modelers get advanced test shots mailed to them to determine any issues with the model kit or instructions. These are often the dreaded "rivet counters" who serve a very useful niche in our hobby. Many other modelers get free kits mailed to them in order to do online reviews of the model kits or items.
So before most modelers can even get their hands on a specific model kit, there are many that have had the kits, gone over them with a fine toothed comb and posted their reviews online. Then those guys often belong to model clubs. They bring those kits into the club meetings so other members will see the kit "in the flesh" and often post short opinions on the kit; the "looks good with some certain details" type comments, etc.
One of the first things many reviewers will post is that it is a new tooled kit or a repop of an older kit or even an older kit along with some retooled or new parts. Tamiya often did this with their older Cold War armor kits. They added some new parts or retooled older sprues to bring them up to Gulf War standards. Other times they will retool to back date Cold War kits into Vietnam era kits (like the old M151A2 into a Nam era M151A1).
New kits of popular subjects also create a lot of "buzz" on the internet. So if company "A" announces a new tooled subject, everyone is talking about it. Company "B" then releases their 30 year old kit of the same subject in the hopes that they can siphon off some of the sales from unwary buyers. They think, "well, I've never heard of company A, but I know company B makes awesome kits so I'll buy theirs."
Then they realize they just bought company B's 30 year old kit at full price when you can find it in the clearance bin for $10 or at swap meets for $5. Tamiya is often critiqued for doing this.