How do we learn that Sammy is an experienced girl-watcher in "A & P"?
We are never told this directly in the text. Rather, we have to take what the text tells us and use it to infer this about Sammy. It is the beginning of the story that you want to look at in order to find the answer to your question. In particular, the way that he describes the girls that enter in their bathing suits and gives a very accurate and full description of each of them. Consider the following example:
...and a tall one with black hair that hadn't quite frizzed right and one of these sunburns right across under the eyes, and a chin that was too long--you know, the kind of girl other girls think is very "striking" and "attractive" but never quite makes it, as they very well know, which is why they like her so much...
Clearly this indicates that Sammy spends lots of his time looking at girls. In addition, the way that his descriptions are very physical indicate that he is a normal teenage male with hormones running around his body and that he spends a lot of his time looking at girls and fantasising over them.
Thus the answer to your questions relates to the way in which indirect characterisation is used to tell us more about Sammy. The kind of physical descriptions that he gives of the girls, and in particular the physical focus that they have, clearly indicates that he is an experienced girl-watcher.