I would also suggest watching how the player rolls as well. Sometimes you accidentallycup the die and end up screwing up your own rolls. Players also have sets of dice they love to use all the time, but they might be bad quality due to glitter, air bubbles or non standard die format (aka spin downs or ones that done add up to 21 on opposite ends added together.) We have a player that seems to always roll below average numbers, one will always roll a 1 on a Fort save, and I roll high low consistently. My dice are 16 years old, are non standard format and have been nicknamed by everyone "PC Killer."