Q: How does a mechanic know the timing belt is broken

The mechanic told me that the timing belt broke on my 1999 Honda Accord (although it was changed 50K ago). How does he know it is broken (with the concomitant likely engine damage) as opposed to slipping badly (big oil leak under timing cover which is supposedly what led to the timing belt breaking)? Car briefly hesitated twice on interstate and I got off the road onto the shoulder and stopped I mmediately; car would not restart; towed to shop that replaced the timing belt 50K ago. Water pump and balance shaft belt etc changed then too - but they did not change the cam and crank seals etc. at the same time. I now know I should have that had done then too but I did not know about the seals and they did not tell me about them. 242000 miles on the car; 3rd timing belt (recommended change every 100000 or so miles). Honda tells me high likelihood of valve/piston damage if belt broke. Car otherwise in good condition for a 1999 car.