The presence or absence of ketones in your blood can tell you whether or not (say) that coffee you just drank took you out of the fasted state (that you were in for not eating overnight).
Whether or not you are in a fasted state matters not just for weight loss, but also for things like longevity (when you are in a fasted state your body cleans up dead cells that contribute to things like cancer, whereas in a non-fasted state it leaves them lying around).
A cup of coffee has zero carbohydrates, trace lipids, and about half a gram of protein. That's not going to pull anyone out of a fasting state.
Now, if you're talking about a cup of flavoured milk, that's a different question...
People tend to add single or double cream (1/2 & 1/2 or whipping / heavy cream, respectively, in the US) to coffee, both of which have less carbohydrate, and generally add a tablespoon or two (15--30 ml) per cup of coffee. The carbohydrate content is < 2g.
That's unlikely to bring someone out of a ketosis state, which requires a daily intake of < 15--30g. That's about two full cups (450 ml) of single cream.