There is no foolproof answer or easy way to do this. The only real way to do it is for the people to constantly be engaged and paying attention to what the government is doing. The media is a hugely important piece in this because they have to practice "watchdog journalism" in order to make this happen.
The only way people can know if their leaders are misleading them is if the people get good quality information and if they pay attention to it. The media must gather facts about what is truly going on and must report it alongside with what the leaders say. Then the people must make sure that they pay attention so they know if their leaders are misleading them. Finally, they have to act on the information and throw their leaders out of office (by voting) if they mislead the people.
Someone once said that eternal vigilance is the price of liberty. That is the case here. People can only avoid being misled and manipulated if they are constantly vigilant.