Bhava Tanha & Vibhava Tanha

The above mentioned technical terms are respectively translated as Craving for Becoming (something) & Craving for Non-Becoming/Getting Rid Off. Above said, how do we know that we're in Craving for Becoming (or not) whenever we're cultivating good? One of the limbs of the N8P, that is, 'bhavana', is about active cultivation. Where is the difference between the two? Similarly, if we do virtuous acts and reflect on those acts regularly, the perception of a 'virtuous person' increases as well, doesn't it? This too, then, is Bhava Tanha? Now with Vibhava Tanha: Suppose I am overweight and must lose weight immeditately; is the 'getting rid' of my body fat then considered vibhava tanha and at the same time 'bhava tanha' (becoming a skinny person)? Finally, when do we exactly know whether something is bhava tanha or kusala chanda? Is it primarily our motivation, i.e., in this case, for whom we're losing weight/what motivation causes us to lose weight, or is it something that we can't avoid at all, since 'identity birth(s)' ('I am this', 'I am not this') exists in everyone except in a fully enlightened being.