Answer: You don't.
But that's not going to stop a newly formed organization created by Aomori citizens from trying. Mainichi says that the ladies are sick and tired of the looky-loos at their pride and joy: Sukayu, one of the Japan's most famous hot spring resorts. The problem here is that it has always been a mixed bath, meaning that the starers and the stare-ees bathe together.
The Konyoku wo Mamoru Kai (Protect Mixed Bathing Association) wants to somehow stop guys from staring at the gals in the bath. But what are they going to do? Blindfold them? The Sukayu Hot Springs are said to be over 320 years old. By blocking the stares of pervs, they would be breaking an old tradition! Isn't that bad?
I don't think they get it: if this bath is 320 years old and still popular, it's probably because these guys know they can get clean and sneak a peek at the same time.
Mainichi says that Sukayu has signs for men's and women's bathing areas, but there is nothing there to actually separate them.
What about you? Would you look? Isn't that the whole purpose of going?