In the past I’ve always known, because I pretty much use ubuntu, centOS, or raspbian (name implies debian) so these are all easy. However, I’m working with my beaglebone and it uses something called Angstrom. Turns out there a great resource for linux distros and seeing where they came from here:
http://upload.wikimedia.org/wikipedia/commons/1/1b/Linux_Distribution_Timeline.svg
