Stores post their refund policies in a variety of places, such around the checkout space of the physical stores. Online stores usually link to it in the footers of their websites or in the help sections. Some stores print it on the back of their receipts.
Many states have established guidelines around refund policies. If a store does not have it posted visibly, a clerk or store manager may be able to pull it up or explain it.
If it's not found prominently online, it's a good idea to contact someone from the company, either via a form or a phone number. Online stores that do not have a refund policy established may not be creditable, so if the store is not able to produce this information it may be a good idea to avoid shopping there.