The way a typical heater core goes bad is that it will become clogged overtime with rust or other particles that could be in your engine coolant.
A heater core could also develop a small pinhole leak that lets air into the system which prevents the coolant from going through the heater hoses and heater core properly causing your heater to not put out any heat.
If your heat isn't as hot as it used to be in your car or vehicle then your heater core is likely bad or if you notice wet spots on the floor under the passenger side of your vehicles floorboard your heater core is bad and needs replaced.
Also if you're not getting any heat at all and your thermostat is good as well as no leaking hoses and proper engine coolant level then your heater core is likely the culprit.