From experience i will say print quality gives even the best knock offs away. Also you can tell reproduction flash rom carts by their (slightly) lighter weight. As for repro mega drive snap cases, they look too glossy and cheap. Alas all of that isn't that evident from (ebay) pictures at first glance.
The larger issue I'm talking about is repros done using donor boards and shells with new labels. Those are nearly impossible to spot from photos off ebay and can be nearly as difficult to spot even when you are holding them in your hand.
I will usually ask for more photos if I'm not sure about a game on places like ebay and also like others, consider the price, user and location of the game before making the purchase. Otherwise, if you can inspect the cart, one thing I have checked to spot some Repros are the PINs of the cartridges before opening it up. Unless it is a donor cart or a Japanese cart, the pins of every official cart from what I've seen has looked the same where Repros I've seen have some oddities or different PIN shapes. Though always best to open the cart up to be safe. Japanese import cartridges though are a pain since to confirm the authenticity of the cart, you will need to damage the backlabel to get to the screws which I avoid at all costs.
All Sega released real carts have crappy textolite PCBs rather than nice fiberglass ones, easily seen by their non transparent brown color. Contacts are always gold plated and edgemost contacts always have a cutout on them on the back. You can see what's in the cartridge by not needing to open it, the shell bends enough that you get a good enough glance of its inners when pulling the halves apart on the bottom. EPROM or some other thing will be very easy to spot that way.