Well, not an easy job, because there are versions even AMONG those original vintage weapons. For example the rebel commando's gun is available in two different colors - light cappuccino and light sand grey - or the vibro weapon for the Weequay and Lando skiff guards is also in two versions. One was used for Lando in ROTJ carded figures AND for the Trilogo Klaatu, I know this clearly because I have them carded as well! In the case of this weapon NOT ONLY the color, but also the sculpting and molding IS DIFFERENT. It is kind of risky, because the second (Trilogo) version is darker grey, shinier and a bit thinner, SO it looks like a repro at first sight! But it is NOT. I also have one REPRO version of this weapon: it is really not easy to tell the difference between the trilogo and this, but the "REAL" repro is even shinier, gleamier and a bit darker, than the Trilogo release. The BEST way of identification is to look at the little stripes at the bottom of the weapon's rod. There is a part of the rod, which is thicker than the rest of it. There are two narrow stripes on it. NOW, in the case of the Trilogo weapon they are close to each other in the MIDDLE of this thick part. While on a repro the two narrow stripes divide the thick part into three, more or less EQUAL parts.
This is one piece of advice for you. Otherwise the 'ping' test is really good, it works usually, because the sound of the original and repro weapon is always different, and the repro makes a louder, PING! sound if you drop it on the table, while the original makes a softer sound. ALSO, the repros are ALMOST ALWAYS significantly GLEAMIER (more gleaming), THINNER and HARDER - less flexible - than the originals. ALSO, the details are more precise on the repro weapons.
I hope I could help you.
Any questions? : ) Don't hesitate.