There are several ways to do this. Sometimes the answer is way too big, or way too small, or has the wrong number of decimal places. If it looks reasonable, then we check the work. One way is to re-do the arithmetic. This doesn't always catch our errors, as we may make the same mistake, in the same place.
Humans are prone to that kind of mistake.
A better way is to re-do the arithmetic, in a different order. Add a column of numbers, from bottom to top. Or, have someone else do the arithmetic. If our answer is 9, then that becomes 0. As a short cut, we don't have to add in any of the 9's in our work, as these are the equivalent of 0.