Over 50 grams of carbs sends me out of ketosis at my current calorie levels, but not everyone is the same. There does seem to be a transitional state where I am 'on the brink of ketosis' or perhaps 'in and out of ketosis', but I much prefer deep ketosis, when I can taste it in the back of my mouth.
You can go to a pharmacy and get ketostix to test whether or not you're in ketosis.
I think we oversimplify the discussion of burning sugar and burning fat. I know I do because that simple explanation is all I care about. I've juice fasted which means I had a lot of sugar, but the calories were so low that I went into ketosis and burned a ton of fat when the sugar was exhausted. I was fat and protein free and in deep ketosis, just based on calorie restriction. So it isn't as simple as it is often explained, and I agree, in your case the restricted calories will have plenty to do with it if carbs are around 80-90.
One possible caveat to your carb and calorie level, is that without enough protein you may burn muscle instead of or in addition to fat. I burned both juice fasting, and now I prefer to go a little slower with the weight loss and keep the muscle. Being as you say, in slight dietary ketosis sounds like a recipe for burning some muscle you'd be better off keeping.
Here's Dr. Eades on topic:
Dietary protein is converted to glucose via gluconeogenesis, which is carried out in the liver. The liver cells don’t use ketones as fuel, so ketones don’t fuel the process of converting protein to glucose. Also ketones are not necessarily “made from body fat when dietary fat runs low”; ketones are produced from fat when carbohydrate intake runs low. A low carb intake stimulates the production of ketones in an effort to replace glucose with another water-soluble fuel.
Reducing carb intake makes the body burn fat and produce ketones. The substrate for gluconeogenesis is protein, and, like the bones are the reservoir of calcium, muscle and other protein structures are the reservoir for glucose. So maintaining an adequate protein intake ensures that you have plenty of substrate for gluconeogenesis without having to catabolize muscle mass to make sugar.