To determine if a putt is against the grain on a golf green, Observe the grass's appearance, where it may appear lighter or darker depending on its growth direction. Against the grain, it may seem darker, and with the grain, it may look lighter. Check for footprints or marks left by previous golfers as they follow the grain's direction. Use a golf ball and gently push it in different directions to observe how it rolls; if it moves slower and resists in a particular direction, it may be against the grain.
Remember, accurate grain reading requires practice and experience, so be attentive and patient to improve your ability to identify whether a putt is with or against the grain over time.