Another "trick" I taught my grand daughters years ago (so that they could appear smart in front of friends) is -- with a sharp eye - look at the bow ....... doesn't matter if you can read it or not ... the Sophie has two (2) lines of print -- bow numbers and U.S. Mailboat -- on her bow whereas Doris only has one -- bow numbers!!
There are other distinguishing aspects, like sloped back window versus not... but those are harder to discern.
In that way you do not have to wait till she passes and read her stern
.