If your fridge appears to have a lot of moisture in it, or is too hot, or is too cold, or too loud, then it may be on its way out.
Too much moisture, or condensation, can cause mould to grow inside the fridge. Too much heat can be a sign of an overworked motor coming to the end of its life. The same could be said for a refrigeration unit that is running much louder than normal. If your freezer is not defrosting (is full of ice) then there could be a problem with how it regulates temperature.
Another big sign that a lot of people tend to notice early on is that their food spoils much quicker in a broken fridge freezer.