The problem with using urine test strips is that they test the waste ketones that you are weeing out. Once you are actually using those ketones (i.e. burning fat) you may find as I did that the test strips stop registering anything. They are generally considered to be the least reliable option, breathalysers such as the Ketonix are more reliable, but if you can afford the test strips then testing blood ketones using a meter is the most accurate method.
I've also used the original/early basic version of the Ketonix, which was much cheaper then than they are now. It basically uses a a coloured "traffic lights" system to check acetone levels in your breath, and has been accurate enough for what I wanted, and it's lasted me for six years for an initial one-off cost of £75 if I remember correctly! But I would definitely think twice about buying one of the current sophisticated models because of high initial costs. I've also more recently used the Libre reader that came as part of the kit with my original sensors, as it's a dual glucose and ketone monitor.
But you don't necessarily need any of these to check if you're burning fat, sometimes you can actually taste or smell "ketone breath", and you certainly should feel far less hungry and be able to go much longer without meals or snacking, have more energy and more mental clarity... All benefits that tend to be associated with a low carb diet anyway.
