Symptoms
Usually none. That is the point: a belt near the end of its life gives little warning, which is why the interval exists rather than a symptom to watch for.
Likely cause
The belt is a rubber-and-cord component that ages whether or not the car is driven. The tensioner, idler pulleys and often the water pump are driven by the same belt and are replaced as a set for that reason.
How should it be checked?
Find the invoice. On a car near or past the stated interval with no record of the work, treat the replacement as due — the cost of doing it early is a fraction of the cost of not doing it.
How can you check it yourself?
Ask directly for the invoice showing the belt, tensioner and water pump. A verbal answer is not a record.
The question to ask the shop
At what mileage and date was the timing belt kit replaced, and did it include the tensioner and water pump?