Long before the days of liquids bans (before the TSA even), I'd seen airport security in some places (particularly outside the US) tell passengers to take a swig of liquids they were carrying on if they weren't factory sealed. And even back home they used to "look through" my SLR camera and sometimes made me turn on my laptop (the TSA went through a brief phase of its own with the whole "turn on your laptop to prove it's really a laptop" thing before giving up on it).

The TSA, however, has it in bold on their website that "you or your infant or toddler will not be asked to test or taste breast milk, formula, or juice."