The short answer
Look at where they stop. If people drop at the same screen every time, it is an onboarding problem and usually a specific one: a required field they do not have the answer to, a permission they cannot grant, or a step that asks for effort before showing any payoff. If they leak evenly across every step, it is a product problem, and no amount of tooltips will fix it, because they simply are not interested enough to keep going. There is a third case worth naming: people who skip the flow entirely and then use the product happily, which means the onboarding is in the way and shortening it will raise your numbers. Watch ten session recordings before you form a theory, since the answer is usually visible in the first three.