You don't, you just think it's improvement and a couple of weeks pass and you think back and look at your maxed out cards and burned bridges and say to yourself "oh."

Jokes aside, there are usually signs you can look for -- sleep is a major one. Can you power through a couple of days with minimal to no sleep, or below average sleep? How many hours of sleep do you get per night in the week? Etc... This is usually a pretty good telltale. Also obsession: are you suddenly just obsessed with something, be it a product you really want out of the blue and dream about day and night, or you want to make a major change to your life, or start a project of some sort.