When you feel a ballotable baby or a bulgy BOW, you will know it.
A bulgy BOW feels like you can imagine a balloon filled with water, with a baseball or something in it (as the head), would feel. You'll feel a soft tension, then after your finger "dents" in a half inch or so, you'll bump against the firm head. Then if the head floats away a little when you bump it, that means it is ballotable.
Don't worry, you won't be able to feel the bag on every patient. Really it's not noticable unless it is bulging a little.
Just give it some time, you'll be fine!