Many problems can be checked by plugging the answer back in. Students are often too lazy to do this. Problems that can’t be checked can generally at least be checked against one’s common sense.
For example if the problem involves finding the time for light to reach us from the moon given certain parameters and you get 1000 years, perhaps you might think there is something wrong rather than just writing it down. Other problems can be checked against the extremes - at really large X I expect the answer to get really small, etc. Don’t waste any of your allotted time. Always use it to check your answers in every way you can.