Basically young stars have a different kind of emission to old stars. You can essentially count up the amount of light from young stars and work out how much star formation you need to have that population.
HST images were needed to investigate the morphology - the shapes just couldn't be picked out in the original images as the galaxies are so compact. However, it looks like a number of them have complex shapes hinting that they are or have recently been involved in mergers with other galaxies. We don't have much to go on at the moment though.
The peas were not named by a scientist.
It's not quite so straightforward due to the complexities of how the peas are actually selected, I think. http://arxiv.org/pdf/0907.4155v1 is the paper - section 2 and 5 might be of interest with respect to this sort of question.
"In the face of entropy and nothingness, you kind of have to pretend it's not there if you want to keep writing good code." -- Karl Lehenbauer
