Triangle should be barely visible in some lighting.
This is not my original work. All credits to Game Verifying.
How NTR-005 cartridges should look:
(Specifically Platinum)
Label should be centered and have a noticeable margin (0.5mm) from label to cart, but can shift over time.
ESRB rating should have the correct typeface.
Front should have more definition, and clearer.
Nintendo logo background should be translucent. The Nintendo logo background should not be opaque.
Lower-left corner should be straight-cut. It should not be curved.
Font should look slim and blocky in most cases, this can differ for some JP and US cartridges. It should NOT look like Calibri.
Triangle should be barely visible in some lighting. It should not be too deep.
Back should be minutely but visibly concave. It should not be visibly convex.
Injection mold should be faintly visible and shallow. It should not be deep or clearly visible.
Nintendo logo should be clearly definable, matching Nintendo's typeface. It shouldn't be thick, blocky, or not level.
NTR-005 font should not be thick. NTR-005 font should be more defined with a shallow debossing.
PAT. PEND look be more defined with a shallow debossing.
Dot-matrix stamp should look slim like this cart rear below: (different numbers are common, XXXXN0J09 is a legitimate stamp variation).
Kerning, position and size of DB should look like this cart rear below:
PCB colour should be dark green rather than a solid green.
Tab should be chamfered at a 45° angle. It should not be curved.
In the example below, it uses C29-01 as a PCB variation. Note that C29-01 is not a valid PCB variation.
Here is a photo of a fake vs real Platinum cartridge. Remember fake cartridges can vary, so use the confirmed real one for reference. Likewise, don’t assume it’s real because it looks real.

Before and after purchase, post a picture on r/gameverifying, front and back pictures only, to get it verified. If you post pictures featuring debunked methods, like the top of the cart or light method, it will be removed.
Please see:
https://gameverifying.com/wiki/cart-based-systems/nds for more details.