Butterfat content is a big “tell” - American butter has to be at 80% butterfat content to be considered real butter, while European butters are usually 82% or slightly more, I think. American butters are fine for most baking where butter isn’t going to be a discernible flavor, like brownies.
Also, whether the butter is cultured or not. Cultured butter will have more of a tang to it.
