How to make sure that the work you have done (coding, design, writing or any other on that matter) is as per the quality which you hoped for. I have found that it is very difficult to find small errors even if we go through the work we have just done. Hence we normally ask some one else to review our work. But if that is not possible the best alternative is to keep it on hold before making live for at least 1 day and then review it again. By doing this you will find many small mistakes which was hidden to you while creating it. This is because while creating things our creative side of the brain is active and we cannot flip to analytical side quickly. Hence after completing our work brain is still stuck on creative side so it cannot judge for mistakes.
Try it out and let me know if it works for you too.