After wanking and shooting your load into a tissue, you make the fatal mistake of touching the tissue to your still wet and sticky cock. this causes the tissue to absorb some of your cum and, as wet paper does, separate. On removal of the tissue you may find small scraps still clinging to flesh. These scraps must be removed manually whether at the time of the offense or some time later (when they have gone hard!)
It is a tell tale sign you have been wanking.
"Bloody wanker's helmet" he muttered as he picked the wet flecks off his purple bulldog.
After going to the loo back at her place he was horrified to find he had wanker's helmet