Ideally, audition pianists* will ask a singer, "What tempo do you take this at?" and then hear the first line sung as the vocalist likes to perform it, with a clear and steady tempo. Ideally. Realistically, however, I often deal with a student suffering from copious amounts of adrenaline. This turns the simple matter of ascertaining the correct tempo into a complete crapshoot.
Very young singers, for example, when asked about tempo, may respond with: "silence . . . (blinking)." I have also heard, "Oh, the usual tempo" and "A medium tempo, please", which is essentially the verbal equivalent of a shrug. The ''Could you sing the first line for me?" route is generally more successful, but sometimes the instruction is interpreted as "What are the first 8 pitches of your song?", which are then sung without a trace of their actual rhythmic values. Or sometimes they will rattle off their song at a breakneck speed, as if you asked "Remind me how the song goes." There's really no way to win, sometimes.
Situations like this have led me to fantasize about carrying a huge bag of metronomes among singers, handing them out like candy. The other solution I've come up with is to learn every song ever written, complete with common interpretations.
But in the meantime, I'll just deal.
*Or any pianist functioning in similar 'little-to-no-rehearsal' situations