You have a good point that it's very hard to define a shortage by lack of applications, when there is always a certain amount of churn.

It would be much better to simply define this as an economic test:

1) Are the prevailing wages for a position rising faster than inflation?

2) Is unemployment in the position both low and remaining flat?

Seems hard to claim a shortage if either of those is false, and there is little gaming to be done for simple government reported labor statistics, recent news notwithstanding.