It might seem like the PE ratio is very useful, but it's actually pretty useless as a measure used to make buy or sell decisions, and taken largely on its own, pretty useless becomes utterly and completely useless.
Stocks trade at prices based on future expectations and speculation, so that means if traders expect a company to double its profits next year, the share price could easily double (there are reasons it might not increase so much, and there are reasons it could increase even more than that, but that's not the point). The Price is now double, but the Earnings is still the same, so the PE ratio is double, and this doubling is based on something some traders know, or think they know, but other traders might not know or not believe!
Once you understand that, what use is a PE ratio really? The PE ratio of a company might be low because it is in a death spiral, with many traders believing it will report lower and lower profits in years to come, and the lower the PE ratio of a given company gets probably, relatively, the more likely it is to go bust!
If you buy a stock with a low PE ratio you must do so because you feel you understand the company, understand why the market is viewing it negatively, believe that the negativity is wrong or over done, and believe that it will turn around.
Equally a PE ratio might be high, but be an excellent buy still because it has excellent growth prospects and potential even beyond what is priced in already! Lets face it, SOMEONE has been buying at the price that's put that PE ratio where is is, right? They might be wrong of course, or not! Or they might be justified now but circumstances might change before earnings ever reach the current priced in expectation. You'll know next year probably!
To answer your actual question... first you should now understand there is no such thing as a stock that is on sale, just stocks that are priced broadly according to the markets consensus on its value in years to come, the closest thing being a stock that is 'over sold' (but one man's 'over sold' is another man's train crash remember)... so what to actually look for? The only way to (on average) make good buy and sell decisions is to know about investing and trading (buy some books, I have 12), understand the businesses you propose to invest in and understand their market(s) (which may also mean understanding national and international economics somewhat).