What is the simplest way to tell if my pool has a leak or is just evaporating?

Try the bucket test first. Fill a bucket with pool water, set it on the pool step so water levels match, mark both levels, and run your pump for 24 hours. If the pool level drops more than the bucket, you have a leak; if they drop the same, it’s evaporation. A normal pool loses about 1/4 inch per day in hot, dry, windy conditions.