Random effects are those that use the word 'random' or involve rolling a die, e.g. Salmon of Knowledge. To determine a random outcome, use any reasonable method, such as rolling a die, assigning equal probability to each possible outcome. For example, if there are three units at the location you target with Lightning Bolt, you could assign 1-2 to the first unit, 3-4 to the second unit, and 5-6 to the third unit, then roll a six-sided die.