The best way to know that your blades need sharpening is to check the grass after mowing. If it doesn’t look smooth, it is a sign. Also, you may look at the blades themself, and if they have large chips or dents, you should sharpen them. But if you sharpen your blades twice a season, you won’t have any problem.
When you see that blades have become thin, you should replace them for sure because such lawn mowers can cause severe injures to you and your kids.