Did it happen one day, suddenly...Or was it a slower process, of shorter naps? Or occasionally missing a nap?

I am trying to figure out if LO2 is trying to drop her nap or not. She is going to be 2 next month, so I am hoping not. Her sister dropped naps around 18 months, that was brutal. Monday she took over 30 minutes to fall asleep, yesterday she took a 40 minute nap and today she didn't nap at all, I left her in bed for an hour and a half and nothing. I'm hoping if I just keep putting her up there she will continue to nap.