So, if you're fond of fundamental analysis then you must conduct a full analysis of the particular company which you consider to buy stocks, if you're fond of technical analysis and short-term trading activity, then you should just learn and practice properly all the patterns/indicators and so on. In my opinion, traders always have to refer to the history of company, it's profitability and annualy incomes, look at the dynamics of stocks price change. If you see that a certain company has a great increase in stocks price for the last 5 years, then you might be sure that it still continues its way to high. But, don't be too confident.