Someone do a DNA test, because Mary-Kate and Ashley Olsen are supposedly fraternal — aka, not identical — twins. Madness. (That means their song “Identical Twins” off their hit record, Brother For Sale, was a lie.)

These birthday girls, who turn 28 this Sunday, July 13, certainly look enough alike to pass for each other, and we have a hard time telling who is who. After some scientific calculations; many years of watching Full HousePassport to Paris, and So Little Time; and a little help from this video, we think we may have cracked the code on how to tell the Olsen twins apart.

Check their hands.

Credit: Anthony Harvey/Getty Images Photo: Ashley (L) and Mary-Kate Olsen on May 7, 2002

Mary-Kate is left-handed; Ashley is right-handed. If you see a twin holding a gargantuan coffee in her left hand, it’s likely MK.

Find a freckle.

Credit: Lorimar Television

It’s often hard to see, but if you’re watching reruns of Full House, you might spot the tiny freckle Ashley has under her nose. According to her, it disappeared when she got older.

Gaze into their eyes.

Ashley’s peepers pop, while Mary-Kate’s aren’t quite as wide.

The nose knows.

Credit: Stephen Lovekin/Getty Images for Mercedes-Benz Fashion Week Photo: Ashley (L) and Mary-Kate Olsen at the J. Mendel Fall 2012 NYFW Show

It can be hard to bust out the eyeball trick if MK and Ash have donned sunglasses, as they so often do. Fortunately for us, Mary-Kate has a slightly slimmer nose than Ashley.
 

Measure the difference.

Credit: Larry Busacca/Getty Images Photo: Mary Kate (L) and Ashley Olsen at the 2012 CFDA Fashion Awards on June 4, 2012

Ashley is taller. By less than an inch, but still.

Browse their brows.

Credit: Taylor Hill/Getty Images Photo: Mary-Kate (L) and Ashley Olsen at the Lexus Design Disrupted Fashion Event on September 5, 2013

This mostly helps the twins are side-by-side, as both girls keep their brows pretty well groomed, but Mary-Kate keeps her brows a bit thinner.

Do a fashion check.

Credit: Andrew H. Walker/Getty Images) Photo: Ashley (L) and Mary-Kate Olsen at the 2011 CFDA Fashion Awards on June 6, 2011

The designers have different styles. While Ashley opts for more simple, feminine ensembles, Mary-Kate piles on the accessories and often wears darker colors, for a slightly edgier look.
 

Look at the lips.

Credit: Frazer Harrison/Getty Images) Photo: Ashley (L) and Mary-Kate Olsen on November 12, 2008

Both twins enjoy making the duck lips, but MK does it more. So if you see one of them doing it, it's a 75% chance it's Mary-Kate. Plus, Ashley has more pillow-y bottom lip.

This article is an update of a previous feature by Jenny McGrath. Follow her on Google+!