Is it a look of fear? Awe? Shock? Admiration? Hunger?
I was getting a massage therapy appointment one time with a lovely young lady, and she asked me to undress down to my underwear (tiny bikini briefs at that time) and then stand in front of her as she was sitting on a stool. I had a huge bulge that was more than obvious. She had a look of nervousness on her face and motioned for me to turn around. It was hilarious because her eyes were glued to it and she didn't blink.