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I would have to ask them if they are where they wan't them to be.

Because success can be very different for everyone.

I would say they don't worry about their next meal, do all the extra-curricular activities they want for their kids if they have them, can travel once or twice a year, they are healthy... things like that.

If you have a lot of money but are always worry about getting more, you have not succeeded

That they woke up today :-)

if they are having fun every day and are on an upward trajectory