The Great Gatsby if you know what I mean

I'd like to know if the phrase "if you know what I mean" in the following excerpt of the second chapter of The Great Gatsby is an idiomatic expression, meaning more or less "I assure you":

“I like your dress,” remarked Mrs. McKee. “I think it’s adorable.” (...) “But it looks wonderful on you, if you know what I mean,” pursued Mrs. McKee. “If Chester could only get you in that pose I think he could make something of it.”

Thank you.