There is some Viceroys that don't have the black line so that mark is not always reliable. There is several other ways to tell the difference though. One is the structure of the cell veins on the hindwing. On the Monarch the large discal cell on the hindwing is closed while it is open on the Viceroy as you can see in this photo. The antennae tips also droop on all the milkweed butterflies and do not on the mimics like this guy.

I've never encountered a viceroy without the black line, since I have rarely encountered viceroys. But because I've seen many monarchs, I have been able to tell that they are indeed monarchs. When I do see the viceroys, it is probably the differences you have mentioned that I notice, but I just never really think about the differences. To me, it is just like looking at people: One person doesn't look the same as another. And you don't necessarily have to compare the people to know that they are different.

it's my first encounter with a Viceroy, so I'm very happy.