K J,
The lactation consultation that I spoke with said that you can tell that you have letdown when the nipple that the baby is not feeding on is exposed, and it begins to drip while you are nursing. There is an exaggerated letdown which is usually more forceful than just dripping (sometimes your baby will cough or choke while nursing). Your milk supply will determine if you can fill bottles.
I, personally, have experienced a tingling-like sensation when I experience letdown.
Congrats on breastfeeding and keep up the good work! It is tough, but SO rewarding. I have been enjoying it for over 8 months.
J.