how can I detect the contamination with acetic acid in a home brewed spirit? I fear the batch might be spoiled.
A preliminary consideration first: it is not a good lab practice to smell unknown substances and this shouldn't be done. With this in mind, is however possible to detect or get a hint of substances on the base of their physical characteristics as well as they particular colour or odour - at least sometimes this is unavoidable.
Obviously a sort of knowledge of the sample in handling is required. For instance, a smell of garlic would naturally indicate Allicin, however the same smell while burning a (in)organic mixture would rather point to As.
To answer your question directly: if you smell acetic acid out of a homemade brew, acetic acid is there. The first test for acetate ion is its displacement from the matrix by adding, say, hydrochloric acid. In your case acetic acid is already free and due to its volatility can be smelled and detected.
Given that you beverage is alcoholic, a nice confirmation test to try would be adding a drop (test tube scale) of conc. Sulphuric acid. Esterification should occur, and the smell should get a sweet note, like in nail enamel removers.
For this and other acetate tests look at https://en.wikibooks.org/wiki/Inorganic_Chemistry/Qualitative_Analysis/Tests_for_anions#The_acetate_ion
By the way, you will find that smell is atop the list.