In this image of a white variety of dog rose, you can see a fringed sepal peeking out at the top of the flowers. Those sepals are the most used way of identifying introduced wild roses. Native flowers mostly have unlobed sepals, aliens are lobed.
Dog rose gives me a chance to talk about another one of those technical things I learned while looking this stuff up. Who was it that used to have a column about things he learned while looking up something else? Anyway, dog roses practice permanent odd polyploidy. What a fun phrase! Remember your junior high biology, and all those paired chromosomes? Well, dog rose doesn't. Only seven of its many chromosomes pair up. This results in chromosomes that are fully present in ovaries, but only seven of them in pollen. Why? I don't know. It doesn't seem like a well thought out strategy. But apparently it works.