People seem to like lists so here are a few lists. The first one is inspired by a question I had asked on a forum some time ago, second was seen on a forum rather recently, third, fourth, and fifth were at the request of someone who emailed me asking the questions.
List of people who saw the Neko-ken in the manga:
Some school people.
Some people on a beach.
Ryoga (might not have actually seen it but was there in pig form when the phobia was explained)
People of note who didn't see it: Ukyo, Kodachi, Ranma's mom, Mousse, Miss Hinako, the principal, Happosai
List of people who know of Ryoga's curse in the manga:
People of note who do not:
Akane, Ukyo, Ranma's mom, all Kuno's, Miss Hinako, Gosugunki
List of people who know of Ranma's curse in the manga:
pretty much everyone except Kodachi (only saw it occur once in the manga and might have rationalized that it didn't happen since she has not seen any other magic or seen it repeatedly), Tatewaki Kuno (who has seen it repeatedly and should know but doesn't), and people who are not near Ranma often (for example Mikado)
People who actually saw the senken techniques:
People who should have no knowledge of them: Pretty much every other character in the manga baring Nabiki and Kasumi who might know they exist but did not see them used and therefor do not know the majority of there effects. Ranma's mother might because she used one of the scrolls to make envelopes but nothing in the manga indicates she does.
Top 5 characters which used magic or magic items the most often in the manga:
Did not use magic as often as they are portrayed to do so in fanfics:
Cologne - she has access to magic and gave magic items to Ranma, Shampoo, and Akane to use but she rarely uses it. The above mentioned 5 use it far more often in the manga than she does. The only time she really uses it in the manga, when not as an example of how to use an item, is the noodle of strength and that was to sell terrible noodles she purchased.
Gosugunki - Attempts Japanese voodoo without success and one time purchased magical mind control paper (battle armor was technology based not magic).