I too have wondered this, (as SLF), how hard do you have to tug on the mask to get oxygen flowing? Considering the noise and confusion, this lack of knowledge, coupled with there probably being no indication as to if oxygen is flowing seems like a flaw to me.
I would probably be so worried that I would damage the mask system that I would give it only the most gentle of tugs! I'm not too concerned though , because it seems you can go without for long enough to get to ~8000.
My parents, who supposedly listen to the saftey briefing didnt know you had to tug on the mask at all.