Without maintenance history of a car, the best possible way is to check CarFax for a repair history report, ask the previous owner, or look for a sticker under the hood indicating it has been changed. If all these fail, best course is to have a technician inspect it. Some OEMs recommend replacement as early as 30,000 miles, while others suggest maintenance around 100,000 miles. While timing belts are relatively inexpensive, if one breaks while the engine is running, it could cost thousands of dollars in damage so it’s important to be proactive. The best course of action is to change the belt if you cannot verify that it has been replaced at the OE suggested intervals.