Low carb bloggers seem to be paranoid about the level of beta hydroxybutyrate in their blood. This is one of the markers of ketosis (it's not actually a ketone, but everyone seems to forgive it for that), and authors of various keto books and blogs suggest you "need" a level of between 0.5 and 3.0 mMol/L to become "keto-adapted" - ie, able to subsist on ketones alone. If you are on a traditional high-carb diet, there will be bugger-all - 0.1mMol/L or less, because this stuff really really hates insulin.
Blood monitors are cheap (you use the same ones as diabetics use for glucose testing), but the test strips are bloody spendy, so I don't test often; the obsessives test at least once a day - I'm happy with once a week.
It turns out that getting a BoHB level above 0.5 can be quite elusive. Mine has lurked in this region, whereas boab seems to have taken to it much better, with levels of 1.4/1.7. I suspect she has much better control over hidden carbs than me.
For example, I drink tea. A lot of tea. I had forgotten to log the milk - which of course, has 6g/100ml of lactose. I was probably having close to 50g of lactose a day, just from milk in tea. So I'm in the process of testing tea with milk alternatives - currently almond milk. It seems OK - makes the tea taste a bit nutty (no shit, Sherlock?) but it's better than black tea. I could have coffee as I have it black, but I don't like decaff (it makes me fart) and there's a suggestion that caffeine can raise blood sugar, which in turn triggers insulin and BoHB runs for the hills.
The best way to make this work is to get carbs well down; certainly less than 50g per day, and possibly less than 25g "to be sure". You can still get useful amounts of veggies, salads and fibre in, within that budget - but it means you need to focus on fat for fuel, and those of us who've grown up and middle-aged in the "Fat is BAD!" era, have a lot of brainwashing to turn a blind eye to.