With dd1, I never felt my letdown either. I didn't even know that you could feel it until I went to a mom's group and they were talking about it.
However, with dd2, I had this strange tingling sensation in my breast and then within a few seconds my shirt would be wet. After a couple days of this, I finally caught on that the strange feeling is "letdown". (or I would be nursing on one side adn all fo a sudden feel the tingling in the other breast and be all wet if I didn't switch the baby asap.)
I would first say that you would know what it feels like once you have experienced it a few times. Kind of like feeling hungry - hard to describe but you know what it is.
For me, it feels like a small twinge in one or both of my breasts, that starts deep inside and gets a bit stronger and radiates outward. It doesn't hurt but it is also not the most pleasant feeling. It only last a couple of seconds and within a few seconds my breasts begin to leak.
dd2 is now seven months old and I have found that the strong letdown that would make her choke gradually went away by 4 months. The feeling of letdown now only seems to occur when I haven't nursed dd in several hours.