The knitted loops should be round, rather than elongated or long as if they are being stretched. Often, knit fabrics are stretched during processing. Remember, the rounder the loop, the less shrinkage should occur. Also, check the grain of the garment. Usually the fabric is knit first then the garment pieces are cut out.
Some shirts are circular knit.
The body of the garment forms a tube and there are no seams. With either type of construction, the vertical rows of loops in a knit shirt should be at right angles to the hem. The side seams should fall below the underarm. If side seams or vertical rows of loops twist around the body, the problem will only get worse after washing. Look carefully at the rows of loops.