Unless your particular scope has the ability to trigger on a pulse greater than a particular length, you have to do this externally.
You could do this in hardware with a retriggerable one-shot. Incoming pulses trigger the one-shot. Additional logic sets the trigger output high whenever the one-shot output is low, but the incoming pulse is still high.
Or, if you want to detect long pulses of either polarity, then arrange for the one-shot to be triggered by both edges of the incoming pulse. If the one-shot output ever goes low, then a long interval has been detected.
In either case, this trigger signal is connected to the scope's external trigger input, and of course the scope has to be set up to trigger on that.