if you have a multimeter and a gba or dmg:  plug the cable into your console and turn it on.  look up on interwebs which pins on the end of the link cable are ground and voltage, and then with your multimeter check and see if you read anything.  most multimeters have the somewhat-sharp probe deals, you should be able to check without an issue. 

if you don't read anything, the 5V probably wasn't wired

apart from this, or someone being familiar with this cable type already, you'll prolly have to cut it open to check if 5V is wired.

however, even if it isn't, the mod is achievable you just need to supply the keyboard power otherwise the key presses aren't recognized

I am in the process of building an LSDj keyboard, and had a quick question about link cables. I have 2 cable on me right now, but they are odd. They both have one GBA connector on one end, and then a split to 1 GBA and 1 DMG size connection on the other end.

                 /---- DMG
GBA-------
                 \---- GBA

I dont know if this (these) cable will work for this project, and would hate to junk a cable for no reason.

Any advice?