Some plants or shrubs can be distinguished by specific traits like leaf colour. The Diablo Ninebark with it's purple leaves is hard to miss. So if you get a purple leaved Ninebark and propagate a hundred of them for sale you would be violating the patent.
I am not a patent lawyer but I suspect that it is your intent as much action that means more. If you don't ask questions and propagate with intent to make money that'sthat will be trouble. If you receive a gift which is hard to identify and grow it in your garden it's not worth anyone's time to identify you or prosecute.
@Dmitriy Likhten Yes, farmers have been sued by agro corporations for patent infringements. However from the original question it does not seem that @jmusser will be planting a whole field of plants with intent to sell them for profit.