One way to tell is that the thin slots that the tabs on the cap locks into on the plastic frame should be close to the front edge. Another difference is that the default angle on the swivel between the plastic frame and the metal base will be different to point the mirror to the driver's eyes.
No problem, Sprinty. If you carefully twist the frame on the base until it clicks into a natural detent, you should find the angle the unit wants to be positioned. The neck on the plastic can be fragile and break in that area. So, do that as carefully as you can. That's likely the main reason these side mirrors are becoming in demand and pricey. That should confirm the question even more completely.
When mounted on the doors the driver's side should angle back a bit from 90 degrees to the car, to align with the driver's eyes. The passenger's side angles back at a more extreme angle to accomplish the same result.
The first hint I left in my earlier reply was mostly to help if the mirror was without its motor, glass, and bracket. If you have only the plastic frame and metal base assembly, that clue then comes into play.
I do have some extra repair components (motors, brackets, bases, and caps) if anyone has a good frame to work with.
Also... from working with these mirrors, the wires coming out of the motor appear to be hard-wired in. But with some needle-nose pliers these can actually be carefully wiggled out of the back side of the motor. Not a bad idea to take a photo of where each color wire goes before disassembling. Nearly any assembly or disassembly can be done with no tools. Just using your fingers. In the worst cases, a flat screwdriver can finish the job.
Joe: JB Weld is the usual method for reattaching the mirror to the base. I think the swivelling ability of the mirror is overrated. When you glue the mirror to the base the critical element is getting the mounting angle of the mirror correct. GTV6 mirrors for both sides are hard to find. The best I can offer you is a broken one that is snapped off at the base-just like yours.
I've spoken with people who reported successful results using JB Weld but I haven't tried myself to glue one back together. Here's where I bow to the talents of PlanetMojo and Racing Swim who just might be familiar with this repair(there may even be an old thread on this).
I'm not bad at refinishing the mirror caps and creating one nice complete one from a couple of bad ones... IF I have a decent frame/base assembly to work with. If the plastic frame is broken at the neck, I usually give up at that point and throw my hands in the air. As these are becoming harder to find, I may eventually try the J-B Weld repair on one to see if it seems strong enough to use. Clamping tight around the neck looks like it could be a bit tricky. But some clever thinking on someone's part might solve that. The only option I can come up with is to put a film of Vaseline on the more angled side, build up a block of Bondo there, and file a flat surface parallel to the other side. Then remove the Bondo block and clean when the J-B Weld has set fully?
I have a few of those broken frame/base assemblies that I haven't chucked. So, I may give it a try in the future. Good luck to anyone giving it a shot. Let us know what you came up with.