1. Location of pollen-producing flowers for harvest: wild or cultivated plants
2. Bee-keepers arrangement with farmers use of pesticide in avoiding the collection of pesticide-laced pollen
3. Bee-keepers knowledge & expertise in processing & packing bee pollen - without heat treatment etc
It is almost impossible to find & verify a truly pesticide-free bee pollen product, unless the bee-keeper operates exclusively on the mountains or in areas free from any modern farming. (Himalayas, Alpines, New Zealand & other mountain ranges comes to mind...)
Since the daily consumption of bee pollen is relatively low (1-2 teaspoons), it may be worthwhile to pay a bit more to buy bee pollen from pesticide-free locations.