Value squint
Colour hides tonal mistakes. Reduce an image to a few greys and see if the picture still reads. Your image stays on this device.
Tone share across the image, dark to light:
What to look for
A picture that works usually settles into a clear dark, a clear light and a middle. If almost everything lands in one or two tones, the piece will look flat, however good the colours are. If your focal point sits in the same tone as its surroundings, it will not stand out. These are guides, not rules: a misty scene is meant to be close in tone.
See also all mistakes about tone and contrast, or diagnose by symptom.