I get so frustrated and horny when my clothes don't fit. The pain of the fabric against my skin really gets me going. part of it is the shame, the other part is excitement others will notice.
Hands down, the most blatant evidence of my wife's gain is our yearly Christmas party. She'll always put on last year's outfit and it's always comically small.
Sometimes you can tend to feel like your feedee is not gaining at the pace you'd like. But it's these moments that are eye-opening when you can see she added a good few inches to her waistline even when you thought she had "plateaued".