Shibboleth can reveal if someone belongs to a particular group through specific language use. Her use of local slang was a shibboleth that proved she was a native.
Pro Tip 2/3
Insider Knowledge
Understanding a shibboleth may require insider knowledge exclusive to a certain community. Only a programmer would understand that technical jargon was a shibboleth of the coding community.
Pro Tip 3/3
Subtle Nuance
Some shibboleths are subtle and can easily be missed if you're not very familiar with the language or group. The slight accent on one syllable acted as a shibboleth among the villagers.