It’s minor ketosis reached in short fasts like this. If you’re eating a carb based diet it’ll take around the 24ish hour mark but it varies heavily person to person and exactly how many carbs. If you’re keto or close to keto levels of carbs you’ll be in keto 24/7 or take minimal time to reach it.
When you shed a few lb of water…generally a sign of ketosis.
As far as testing….blood testers. Keto mojo seems popular.
IF will not put you in ketosis. A PF will generally accelerate it. IF in combination with too high of carb macros will keep you out