First of all, they're checking to see that they have enough dried fruit. Now this is not the dehydrated fruit that has given up all its moisture and is easy to handle, but the dried fruit that is really sticky to the touch. It's sometimes quite hard to find at a decent price. The other thing they're checking for is suet. Like the dried fruit, suet is not that hard to come by, but good quality suet is another thing.
Some people can be pretty fussy.
A third item on the check-off list is currants. They're usually not a problem, unless you leave it to the last minute on Saturday afternoon. Then no store has them in stock. All the other ingredients are available at the local grocery store in good supply or at the d panneur. For the uninitiated, this is not a nuclear bomb we're throwing together, but devotees of this particular tradition will argue that it can be almost as devastating, especially after a third helping of turkey with all the trimmings.