A lot of times the plant's tag will tell you. It will say trademarked or patented or will use abbreviations like PVR (plant variety rights). Another option is to identify the plant name via the internet or through a gardening club or plant expert, and then check the database to see if it's listed. It can be a pain. Just in case you guys didn't already know this, unless it's a rare variety, plants are usually cheap to buy wholesale. Lots of people buy wholesale and then retail them. So that's another option if you want to sell houseplants.