well obviously at 150 yards it's going to be hard to tell and in warm weather forget about it. basically if it's cold outside (which 9 out of 10 seasons around here it is 20 degrees or lower) then you can see by their breath when they are exhaling. i shoot the majority of my deer at 75 yards or below because i hunt thick terrain in small openings. i'm just trying to make a point that if you can tell, then try to hit em' when they are exhaling, the results are unbelievable!!!