Ikuko wasn't so foolish to assume she knew from the very first moment, but she had known for awhile. It was something any good mother should have picked up on rather quickly. Her daughter was a super hero. And not just any super hero. Usagi was the leader of the Sailor Scouts, Sailor Moon.
At first she had been worried for Usagi. Not only worried about her safety (she still worried about that), but also worried about what it was that Usagi was up to. Usagi had never been a great student, but she also wasn't a bad kid. Sure she was late to school, but once she was there she always stayed there. Until she didn't. Ms. Haruna called home one day to tell Ikuko that Usagi had skipped class. There was a new girl at the school who had a history of delinquency and she and Usagi and (strangely enough) the school genius had left during lunch and never returned.
That was really the beginning for Ikuko. The bruises that she had noticed recently took on a more sinister meaning. The nights she swore Usagi was too quiet had to mean that she wasn't really in her room after all. The fact that Naru came around less and less was more than just natural growing pains.
But Usagi had always been trustworthy (aside from volunteering her test grades). And Ikuko remembered how much she pulled away when her mother accused her of something she hadn't felt she was guilty of. So she watched. She listened. And she gathered evidence.
But nothing added up to Usagi being a delinquent. Her grades were actually improving, slowly but surely (well, she was hanging out with a genius who had a wonderful work ethic). She was late to school less often. She brought her friends around and they all seemed very polite and well behaved. She helped at her friend's shrine (Ikuko had spied on her, she was ashamed to admit).
And then there were the hushed conversations when Ikuko knew that only Usagi and her cat, Luna, were in the bedroom. Occasionally, the second voice was clear. It was definitely not Usagi's voice.
Ikuko pondered on all of this information for what felt like an eternity before The Event. The Event when it all finally came together. The Event when Usagi had saved her family while on vacation. Ikuko had not seen the her become Sailor Moon, but when she stepped forward to defend the Tsukino family from the youma, it was suddenly crystal clear. Usagi was the legendary Sailor Moon.
Ikuko had not seen Sailor Moon in person before. Perhaps her realization would have come sooner if she had, but at any rate, the realization came suddenly. And even though her daughter was fighting a supernatural being, Ikuko felt a certain peace wash over her. Usagi wasn't a delinquent. She hadn't fallen in with a bad crowd. She wasn't wasting her entire life on video games and comics. She had a gift and she was using it to the best of her abilities.
After all was said and done, when Usagi's friends suddenly showed up at the hot springs, Ikuko had gently swayed her husband to let their daughter drift off with them instead of spending the rest of the night with family. There was something on Usagi's mind. It had been on her mind the entire trip and longer. And it was almost certainly something to do with her secret life. And anyways, the girls had just saved the family. They deserved a small reward.
Ikuko decided then and there that she would do everything in her power to help the girls. She couldn't fight for them, but she could make their lives a little bit easier. When she heard Usagi leave in the night to defend the city from some new creature, Ikuko might check on her in the morning and declare she looked sick and tell her to stay home from school that day. She would pack Usagi a bigger lunch and have snacks waiting for her when she came home. She gave her a few extra dollars here and there to play video games or go out with her friends. And she always welcomed the other girls into her home. Especially Makoto and Minako.
Usagi had mentioned before that Makoto lived alone. So she was always invited over during the holidays. And it sounded like Minako's family wasn't very involved, so she too was invited. And Minako had a cat. A cat that talked, just like Luna.
It took longer for Ikuko to figure out the hushed conversations than anything else. Strangely, it was easier to admit her daughter had a secret identity to fight evil than to allow that a cat could talk. But finally, it was the only conclusion that Ikuko could make. So she watched Luna. She listened. And finally, she tricked her.
Ikuko and Usagi had similar voices. Very similar. So one day just before Usagi was due home, Ikuko ran up the stairs loudly, and just before she reached Usagi's room, started talking to Luna. And the cat answered. Just as Ikuko cleared the doorway, Luna spoke to her and then realized she had been found out. Ikuko just smiled, "I won't tell if you won't tell."
So yes, Ikuko knew her daughter's secret. She knew why she snuck out at night and sometimes fell asleep during class. She knew why she came home with nasty scrapes and bruises. And she knew that, even though strange things were happening in the city and her daughter had to deal with things most full grown adults couldn't even begin to cope with, Usagi was growing up better than Ikuko had ever imagined she could.