How do I know exactly what kind of hydrangea I purchased?

Great question!  Though the containers are similar, there should be a tag on the hydrangea or a sticker on the container listing the specific variety.  If a bar code was scanned, the receipt might have detailed information. If there isn’t a plant tag and the receipt is generic and just says hydrangea, is it starting to show buds or flowers with color? Take a look at the options of Endless Summer Hydrangea flower colors to help match up your plant. You might also check back at your garden center for assistance.