If looking at the box, it will say "Lightstrip Plus", not just "Light Strip". (You have to look somewhat closely at the box, since these are written in a small gray field a little down from the top left) Also, the lights on the Plus box are lit up in blue, where as the original has a green tint to it.
If looking at the lights themselves, the Plus is wider (although if only looking at one, how does this help? lol) So the best way to tell is that the Plus version has a connection point on the non-cord end, so it can be expanded with the Lightstrip Plus Expansion Kit.
I hope this helps!