I used a bright flashlight to illuminate the membrane through the Spandex and it was quite easy to see the wrinkles/ripples in the membrane.
Have another person hold the flashlight while you manage the heat gun. I tried doing both at once and it got quite confusing at times as to which hand had the heat gun vis a vis the flashlight. ;))
You will easily see the wrinkles disappear as the membrane tightens up as the heat is applied, just don’t overdo it.