As far as I can tell, the checker is available to all applicants. The link appears on the left-hand side of the screen at some point when you're clicking through the admissions screens.
It was previously working fine, but as zoosermom notes, it's now acting up (birthday field to be precise). I thought of calling, but we did get a letter confirming everthing was in, so I forgot about it.
I might give them a call and ask what's up.