Remaining cognizant of your proximity to a suspect’s head when you’re standing in front of him (or her!) is obviously extremely important. Monitoring his level of ability to deliver a forward-thrusting headbutt is something which tends to be a regular part of tactical training.
Remember also, though, that it’s just as important to remain aware of your positioning when standing behind a suspect in regard to his ability to deliver a backwards-thrusting headbutt as well.
When standing behind a suspect, be sure you’re far enough away from the back of his head to avoid being smashed in the face should he suddenly thrust his head backwards. Also, be sure to practice countering takedown techniques that can be immediately utilized should he become combative like that while you’re behind that violator.