That's the trade-off with her ability. She can look exactly like them but to act just the way they do, exact mannerisms and tics and all? Raven would have to study that person for a very long time. Luckily, she picks up on things quickly. Emotions are her downfall. If she's new to the form, she would be easy to spot. The way she stands, how she talks. She'll give it a few tries, to see what they see and how they can tell if it's her or not. Raven will already be testing Negan (since he thinks he's so clever) so I imagine that she would do the same to Harley.
I could see the three of them in a room and Raven would get this playful streak (she has them from time to time), shifts into Negan when perhaps Harley's back is turned, change into Negan and then give a good smack on the ass to Harley, then make Harley try to guess which one of them did it.
By the way, I stumbled upon the playlist you have in Harley's journal (beautiful layout, my dear), fell down the rabbit hole and found my way tothis choice piece. I'm sure neither one of our characters probably are good singers but this song suits them still.