Without a camera or emulator. According to some videos on YouTube, this is possible, but they are very secretive. I imagine something similar to RemoteJoy (a custom firmware-only app) for the PSP, but no one wishes to tell.
The big game sites / magazines have some specific hardware to capture video from those consoles. It's very expensive and not easily obtainable for regular consumers, though.
The closest match I could find to the DS LCD is Sharp LQ030B7DD01. You can find a datasheet Here. Aside from RGB, the only LCD signals I needed to use were DCLK1 and GSP1 (everything is conveniently labeled on the DS PCB)
Basically, you will need to tap the signals to the LCD screens and interpret them in order to record video from a DS. To do this you will need some sort of microcontroller setup, and you will have to interface it with a PC. You might ruin a DS in the process. This sort of hack should work with pretty much any portable console.
Otherwise, there might be some software that you can run on the DS that will dump video while playing, but this will interfere with the speed of play, as the DS will need to execute more calculations than it already does.
http://draculaojisan.blog8.fc2.com/blog-entry-440.html
This is a documented version of pinouts and uses a Chameleon USB FX2 board to output video to a computer. I'm waiting and trying to figure out how to do it on a DS Lite, but so far no dice.
If you can't read Japanese, I have the document translated to English.
Otherwise, there might be some software that you can run on the DS that will dump video while playing, but this will interfere with the speed of play, as the DS will need to execute more calculations than it already does.
Like how RemoteJoy is sadly, games were not even playable.
