There should be a colour guide on the side of the bottle. Anything more than 'trace' indicates you're in ketosis.
I don't find them reliable at all, as they only measure 'excess' ketones. Mine are negative a lot of the time, but LT told me this could be because I'm using up all the ketones I produce (i.e.- I have no 'extra' to be wee-ed out!) or they may be flushed out by all the fluids we're intaking. If you're doing this 100%, drinking like you should, and losing weight- then you can take it that you're in ketosis.