One of the signs of a tornado at night would be small, bright blue-green to white flashes at ground level near a thunderstorm as opposed to silver lightning up in the clouds.  This means power lines are being snapped by some pretty strong winds.  And a PERSISTENT lowering from the cloud base, illuminated by lightning.

And whether day or night, there's a loud, continuous roar or rumble, which doesn't fade like thunder.  Some have compared it to the sound of a train.  I've heard it, living near the downtown area when Lubbock was devastated by a tornado  May 11,1970, and IT WAS LOUD!  Here are a few safety tips:   Avoid windows; Get in a shelter or basement; Go to the lowest floor, center of the house; In a mobile home, you're safer outside; If driving in the open outdoors, get out of the car, if there's no shelter,  lie flat and facedown; In a mall, find an interior bathroom or storage room; In a theatre, go to an interior bathroom or hallway away from windows; In a skyrise, try to get to a lower floor and find an area away from windows.