when you give a blowjob can you always tell when the man is about to cum?  
      yea because there this lady and whenever she gives me head or wanks me i never say anything but she always tells me your about to cum aint you! i was getting scared she pyscic or somthing! lol  
      I'm so vocal I think the neighbours know when I'm going to cum  
      Yep.
Usually there is a change in size and of course, there is usually a change in motion.  Yum.
Some bloke held my mouth open with his hands, today.  That was NASTY!  
      Part of the reason she knows is physic  perception