After spending some time in a busy restaurant kitchen, I sort of developed the 'silent timer'. You dump pasta into boiling salted water and then go off to do whatever it is you still need to accomplish for your meal. At some point, something just goes off in your head, and you think 'Oh right! Pasta's boiling... I'll bet it's done!"
I'd like to be able to say I was SO confident in this ability of mine that I simply drain and serve at this point, but I can't lie; I'd feel super-dumb if I served over or undercooked pasta. When my head timer goes off, I take a little nibble and then drain. I haven't been wrong in a really really long time, but I know the first time I don't double check it will be the time I mess it up.