In warm, dry weather, a diesel engine will start even if one glow plug is defective but there will usually be increased emissions because of this and possible knocking during start. Once the weather turns colder and the frosts set in, the heating contribution to the diesel engine fails to function and is likely to produce smoke. It may start with some difficulty but more often than not, won’t work at all.
Federal-Mogul Motorparts has created a useful illustrated diagnostic BERU guide on glow plugs, listing typical damage and related causes. This free guide can be found on page 13 onwards here