Why know the dev name for the phone? You might just be curious? or really need to know since you’re looking for a RUU for your phone, and since those always are listed by just their dev name it could be a good thing to know.
One of the easiest ways to know the developer name is just to look at htcdev.com and under kernel sources for your phone, and just look at a filename there. Like for a htc one, you find M7 in there, which is the developer name for HTC One. Easy, huh?
Or: use the fastboot get var command to get all the info needed about your phone. Where it can be found under Product, in my case M7_UL which means a 4G HTC One.