* If the piston can hit a valve if the timing belt or chain breaks, you have an interference engine. This is the key characteristic. A broken timing belt/chain in an interference engine will cause significant, often catastrophic, damage to the engine as the valves will be bent by the piston.
* Non-interference engines (also called "non-interfering" engines): In these, the piston and valves are designed with enough clearance that even with a broken timing belt/chain, the valves won't be struck by the pistons. Damage might still occur (e.g., to the timing components themselves), but the engine block typically won't suffer catastrophic valve damage.
There's no easy visual inspection to determine if an engine is an interference or non-interference design. You need to consult your vehicle's repair manual or online resources specific to your engine's make, model, and year to find out definitively.
