Age & experience help. Being an avid modeler for 45 of my 50 years on Earth and having used the internet with modeling building since 1995-96, I've noticed trends when new models vs. "new" models (i.e. reboxes/reissues) arrive.
First of all, Google/Bing/Yahoon/Your search engine of choice is your friend. It amazes me that the same person who will drive out of their way to save 5¢ on a gallon of gas (what's that, like a buck on a full tank?) won't do a little online homework before spending $30-50 on a plastic model kit.
One of the things I noticed in my online modeling years is that internet buzz often precedes true new models. Fellow modelers have already received test shots, website moderators have already received review samples and information packets to hype up the new kit. Reboxes/reissues just pop up on the shelf or online retailers' website without much fanfare unless it was a highly sought after hard-to-find kit.
Someone has already bought the "new" old kit and ranted about it at one of the various modeling forums.
One of the various review sites has already received the kit, posted a review with photos and declared it the old XYZ kit. It's fairly easy to recognize if you know that they had that subject kit previously in their line.
Ask at one of the modeling sites. "Hey, I see XYZ has a new M1000 listed, what's it like?" Someone will probably know that XYZ has often released old ABC kits before and ABC once had a poor M1000. Somebody who works at a hobby shop or visited a hobby shop that allows customers to look inside the box has probably already seen and recognized the old kit.
Check the manufacturer's website. When Revell started reissuing the old Renwal kits, they advertised it knowing they were aiming for the nostalgia market. Old dudes like me that built these kits in the 1960s wanting to relive their childhood without buying vintage kits at collector's prices. Round Two has taken to showing the original manufacturer's logo so you know what you're getting.