For me, if a child is still under my roof, I would closely monitor their income and expenditure. If he is spending more than I think he should, it will be the right time to investigate his sources of income. It is also important to always investigate the source of your children's income. Don't rely on what he or she told you.
This is another way to know what your children are doing if they are under your roof, if they are not working you know the amount you give to them and and what they can afford to buy with it if they are spending above the amount you give to them then you should be able to observe then investigate what they do to get the money they spend. Parents should stop believing everything they hear from their children until they investigate and find out things for themselves.