My Sixshot is an original and the copyright on mine reads:

(C) HASBRO .1986
(C) TAKARA CO,.LTD.
1986



An easier way to tell is to look at the box. Obviously the reissue would be in Japanese, but to differentiate it from an original G1 Japanese Sixshot, the reissue should have no tech specs or robot points printed on the back of the box. Where this stuff would be should instead be a white box full of text (legal blurb) including the words "対象年令15才以上" (For Ages 15 and Up). And of course the tech specs bio itself would be on a collectable card.

Also on the bottom of the front of the box you should see these words:
D-98 デストロン/忍者参謀　シックショット　復刻版
("D-98 Decepticon/Ninja Sixshot Reissue Edition")
...so obviously the words in bold are the ones to look out for.

Also the Takara logo will be different. Here is a pic of a MISB Raiden - a Transformer released in the same year as Sixshot in Japan. You can see that the Takara logo on this box features the "black boy" which is absent on the reissue's box. Otherwise the text and font are the same.

Other references:
Images of original Sixshot
Images of reissue Sixshot

I saw the reissue once in 2002 and when I compared it to my original it looked like - to me - that the reissue's green is every so slightly more turqoise than the original. But I'm not entirely sure if this is true or if I'd been staring at those toys too long and my eyes were acting up on me. Does anyone here have both the original and reissue?