If i did, by accident, reverse the polarity of the battery holder, that would effectively murder the mbed, right?
I do not think anything would happen, your mbed would just not run, but dont quote me on that one because I have never tried.
An easy was to check for polarity is using a multimeter. Place one wire on the red and one on the black, and if you get a positive voltage then the one connected to your red is positive, if its a negative voltage then the wire on the red is negative.
If you do not have a multimeter just connect the battery pack to an led and see which one is + and -, if you include a picture of your battery holder I could probably tell you which one is which.
Worst resort is just to follow the logic of the battery holder, track the + charge thought the batteries and see which wire it is connected to.
I know mbed is a prototyping microcontroller (but I'm too cheap to buy another :P), but would it be possible to power the LPC1768 with a battery? I.E the battery goes to a USB out, and that USB out goes into the USB in on the mbed, hence powering it.
Would this be possible? And what voltage and ampage can the mbed take (if anyone knows)?