I'm not calling for tests to confirm some rare condition, I'm calling for tests to possibly disprove (or, since we're dealing with probabilities, make a whole lot less likely) the currently hypothesized condition.

It's exactly this which confirmation bias is all about: It's not that you should be trying to prove anything, it's that you should search for signs that might disprove your currently upheld hypothesis.

Suppose there are a hundred possible conditions to which all have the symptoms X. is your current most likely hypothesis (P>0.5), and the others are equally unlikely. Then the test that you want to perform isn't 99 tests to such that for test , ) is high, you want to conduct only one test s.t. ) is very low, while )) is moderate to high.

I didn't do the actual math here, so am not sufficiently certain (though I am quite confident) that such a test can actually exist (given the priors for contitions and symptoms), but checking for breath alcohol probably is such a test.

Also, please note that in the original post, it hasn't been indicated whether or not the girl has been to a party.

TL;DR: When you hear hooves, think horses, not zebras, but take a breath to check whether they have another feature all horses share, which other animals don't (in this case, perhaps neighing? Might not exclude zebras, but other animals).