Identify the color of the bacteria. Notice the fact that positive-gram bacteria maintain a strong violet color during all phases of the staining treatment. Spot negative-gram bacteria with a change in color -- from a weak violet to pink.
Focus on the decolorizing stage. Notice that positive-gram bacteria are non-resistant to adding ethanol and acetone (the decolorizing agent). Recognize negative-gram bacteria by a loss of all color during the decolorizing (the third) stage.
Observe the bacteria's strength. Pinpoint positive-gram bacteria cells that remain thick and intact during the staining process. Identify negative-gram versions if the cell walls thin out, spread wide or break.