Several ways, but easiest depends on revision levels of your controllers.
Latest revisions (I think it is V19 and upwards), allow you to attach "Connection_Status" to the produced tags, which you can monitor in the consumer(s). It also transports the operational mode, so you can be sure the producer's data is valid (i.e. the processor is in RUN mode.... Producers continue to produce their tags even if the processor is in Program, or Faulted, modes.