Hey there,
Wish this was a simple answer - there are many factors that paly into when a stock goes on sale. It can be trading at a discount to historical or industry averages, or it can be cheap as compared to expected growth rates. In other words, there is no single way of determining when a stock is on sale. The key is to learn how to value different companies, what works for certain industries, etc. Even when you do find a company 'on sale', there may be other factors that are impacting its price - such as macro environment, reputational damage, etc. things that are not readily found in the 'numbers'.
There is no cookie cutter approach to this.
We can certainly help you through the process, have you had a chance to look at our guide around growth stocks? That may help answer some questions. Let me know and we can post the link if you haven't seen it. If there is ever a stock you think is on sale, and would like us to have a look - just let us know and we can certainly look into it and give you our perspective.
Mat