Vocalize a three-tone scale. Start in the extreme low register.
Sing while sliding two octaves without vocalizing, like a siren. The singer should use a "U" sound, like the German umlaut which mixes "U" and "I." This should be done in full voice, meaning that the tongue will not pull back in the throat.
Sing the notes of the scale as high as possible in full voice.
Listen to the high notes. If the voice goes to high B-natural or high C, or even higher, then the singer is a true tenor.
Listen to the timbre. The singer should vocalize a "hum", keeping the tongue between the lips while singing a 5-tone descending scale.
Sing while stretching the back of the throat, keeping it separate from the tongue root while humming. When singing, vowels should originate in the space behind the tongue. The tongue must remain forward in the mouth.
Sing a five tone descending scale on the five vowels while keeping the open vowel space in the back of the throat.
Listen to the vocal timbre. A baritone sounds dark, a tenor will retain many more high overtones on the descending scale.