Whilst indulging in petting/heavy kissing etc you slowly work your hand into your partners knickers and finger her pussy. Then after a minute or so start kissing the side of her neck and at the same time bring your finger up and smell it slyly, if it stinks you know not to go down on her!!
Hey stu did you get anywhere with sarah last night? Yeah man i fucked her but she wouldn't blow me cos i wouldn't go down on her. Why wouldn't you go down on her man? I gave her the sniffcheck and her fanny stank so there was no way i was going down on that.