Everyone has their trick - firstly you need to be intimately familiar with the sound of a particular recording - I like well recorded piano and percussion as the best indicator of tweeter quality (as most tweeters compress all too easily and this is usually the problem). Dave Grusin's Hommage to Duke C-Jam Blues has some hard hitting piano notes. If they sound harsh or grainy or don't sound totally effortless at very loud levels then there might be a problem.
If it is any consolation tweeters are usually the cheapest driver to replace anyway. Replacement part availability should always be one of your considerations in a second hand purchase. Boys will often break their toys.
If it is any consolation tweeters are usually the cheapest driver to replace anyway. Replacement part availability should always be one of your considerations in a second hand purchase. Boys will often break their toys.