if you are going lower than 50g carbs a day on LCHF then your body will normaly go into ketosis and start burning fats for energy instead of the missing carbs. This is quite normal for this diet, and isn't the same as
diabetic ketoacidosis (DKA) the condition that Jack's mentioned, which is very dangerous. This can mainly be a serious problem for type 1 diabetics due to a combination of high glucose levels and lack of sufficient insulin, but may also occasionally affect type 2's if their glucose levels go very high too.
If for any reason you are unable to get the blood ketone test strips prescribed (and they are apparently very expensive to buy and you'll need a meter that will read them too), then you can buy a gadget called the
Ketonix that you breath into to test for ketones; this uses a coloured "traffic lights" system shows none, low, medium or high levels. It should always show your ketone levels, whereas the Ketostix usually only show the waste ketones in your wee as you progress into ketosis. The Ketonix's levels of accuracy are probably somewhere between the low accuracy urine test strips and the high accuracy blood ones.
Robbity