How can I check if timing belt is working properly

If it is running right it is working

Pretty much. Timing belts just make sure the valves (top of engine) are synced up with the pistons (bottom of engine). What I would do is check your engine first. If you have a 4 cylinder engine (I-4) I'd almost be certain you have a timing chain, not a belt. These should never need maintenance or need replacing, unless you do something really stupid (i.e. rev it up to 7000 rpm repeatedly). If you have a V-6 you probably have a timing belt. There also should be a plaque on the engine telling you at what mileage the timing belt was last changed/service (the one on my Pathfinder is right on the front of the engine). Most engines I've seen recommend changing the timing belt every 90,000 to 105,000 miles. If you don't and that belt breaks, you're gonna need a new engine, or a new truck.