Ok, I opened a demo in wolfcam, paused on part of a demo, and did /video jpg, /video tga and /video avi.
I opened the .avi in virtual dub and saved a frame as .bmp which like I said above is uncompressed and lossless.
Now, I don't know any scientific way to tell them apart... but I opened them in Photoshop and did some tests,
here are the results [6MB].
Firstly, the TGA/TGA comparison shows that the method works, the image is completely black, and therefore there is no difference.
The TGA/BMP comparison shows a small amount of difference, this means that TGA export is not exactly the same as AVI export, so the AVI may indeed be completely lossless - unfortunately I didn't printscreen the wolfcam window to confirm this.
The TGA/JPG and BMP/JPG comparisons show lots more difference, showing that the JPEG is actually far lower quality than the AVI or TGA.
So we now know that QL doesn't output the three at exactly the same quality. The quality difference is negligible though so using JPEG output rather than TGA is a better option since it saves loads of space (over 3MB per screenshot in this case), but as explained above when it's converted into an AVI it will make no difference, so using /video avi is the best option as it has the highest quality and is most convenient.