We need more context to explain the meaning of the whole sentence, but the meaning of "guess which" is standard. There must have been reference to two Harley's previously - possibly one of them is a person and the other is a Harley-Davidson motorcycle.
If no one gives you a simple, orthodox explanation, I will go into one of my ruminations about phrasal verbs-- controversially claiming that "to guess which" and "to guess what" are of that class.
Our use of "guess," especially in the imperative mood, is very idiomatic, and it depends on set structures many of whose parts are not overtly stated, but implied.
I'm still chewing on this one-- and when I first read the question I thought it was so simple I had to wonder why you had to ask, as I'm sure many native speakers will also do.
But let's wait for someone to come up with the kind of answer you need, especially if you have test questions that need to be answered correctly-- explanations like that are not my strong point, especially when I see a construction ("guess which x is y") that strike me as mysterious, almost beyond logical explanation.