You can tell how things are working by how you feel. There is an adjustment period, to be sure, but it’s not that hard to get through.
Keto flu is avoidable by increasing salt intake and drinking to thirst. I was forewarned, so I worked at it and never had to experience the “flu.” We enter nutritional ketosis almost immediately, certainly within a few days, but it takes longer for the body’s cells to adjust to using fatty acids and ketone bodies as their primary fuel. We call this adjustment becoming “fat-adapted.” Fat adaptation can take 6-8 weeks, or even longer, and many people continue feeling better and better the longer they are fat adapted.
If you find yourself feeling more energetic, more able to focus mentally, you find yourself losing weight or inches, and so forth, that is a good sign of becoming fat-adapted. Many people find that their blood work comes back into the normal range. My heart rate and blood pressure became normal after decades of being high. The important cholesterol numbers are the ratio of triglycerides to HDL, in terms of risk for heart disease. People eating keto generally also find their LDL falling more into pattern A (low risk of heart disease), even if total LDL increases.
There is no real way to measure fat-adaptation, but the marker we generally use is the level of beta-hydroxybutyrate (one of the three ketone bodies) in your blood. You are most likely to see a level over 0.5 millimoles per deciliter (mmol/dL). The number can be higher or lower than that, but 0.5 is definitely considered nutritional ketosis. (A number higher than that is not likely to bring any greater benefit, in most circumstances.) You don’t need to test, however, if you don’t want to. Some of us do, others don’t. If we keep our carbohydrate under 20 g/day, we pretty much have to be in ketosis, or we’d be dead.