If he sings while reading the lyrics from his mobile phone, best drop him.
If he starts a song at the wrong speed and in the wrong style, and says "Oh, I was doing the Beyonce version", best drop him.
If he sings into the wrong end of a mic boom stand, best drop him.
If he starts searching the mixer for the Autotune function, best drop him.
If he has a persistent sniff, slightly glazed eyes, and strangely jerky movements, best drop him.