Good question , jondar .
Sometimes real fractional (8k,10k,12k,14k,18k ec.) gold content 'findings' are attached to jewelry items that are only non-gold-alloys/plated/junk metal .
In addition , sometimes 'manufacturers' use a karat stamp on things that are intentionally fake (as well as silver/platinum & other stamps) .
About your acid-test question - there are also non-destructive tests for gold content , esp. specific gravity test .
Some jewelers/goldsmiths still use touchstones for gold testing .
Most reputable jewelers are reasonable with their prices on gold testing , but I'd avoid high-end shops , as they seem to charge 'premium' prices for gold testing .
A telephone call to a few local jewelers in your area will establish what they're charging flat-rate , for testing .