If someone had double eyelid surgery, it is not automatic that you will be able to tell if the double eyelid fold is a natural fold or if it is a surgically created fold. This is especially true for people who had suture technique double eyelid surgery.
There are two types of double eyelid surgery. One is suture (non-incision) technique and the other is open (incision) technique. For the suture technique, the fact that incision is not made, there is no or minimal scar. Therefore, the double eyelid fold looks natural and one cannot easily tell if the person had surgery. However for patients who need incision double eyelid surgery (those with excess upper eyelid skin, pre-existing eyelid asymmetry, eyelid ptosis), there are three main signs that double eyelid surgery was performed. One is that when they close their eyes, a scar can be seen. Second, the skin above and below the double eyelid fold looks puffy or swollen. Third, the double eyelid fold does not move with eye opening and closing. What this means is that the double eyelid fold is static. A natural double eyelid fold invaginates inward with eye-opening and the double eyelid fold becomes flat or flush with the skin when the eyes are closed. Thus, with eyes closed, there shouldn’t be depression or an indentation. Therefore, a natural or natural appearing double eyelid fold is dynamic, not static.
So for a suture technique double eyelid surgery, the fold looks natural. And for a typical incision double eyelid surgery, the double eyelid folds look unnatural for the reasons mentioned above. However, if a dynamic incision double eyelid technique is used, then the incision looks and move dynamically like a natural double eyelid fold. Hence, it would be very difficult to tell if someone had double eyelid surgery.