How to know if you need new brakes?

I have a 2012 Honda Civic I bought used in May 2015 with 45 thousand miles. I have 51 thousand miles now and my brakes are stopping but I notice I really have to press my foot on the pedal to brake. I have no idea if its the original brakes either. Also no kind of brake/ warning light has come on. If I need to replace them about how much will it cost?

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