For the Real Die-Hard Mustard Fan it just has to be the Mustard Bag.
How better to advertise your exclusive membership to the Club of Mustard?
When you spot another Mustard Bag holder, approach carefully and after enquiring politely about their health, say, 'Nice weather for the time of year.' Their reply should be, 'Not so bad, all things considered.'
You will have then:
a) Possibly made a new friend
b) Established whether this is a true Fan and fellow Mustard Clubber or just a normal person
c) At the very least, shown yourself to be an accomplished conversationalist