How can you tell if someone has speedhack from screenshots ?

If you ask me, this guy could be pro and knifed 3 noob Ts with knife. The time is on both pics 0:00 (We don't know whether this happened in 2 seconds, if he has speed or not) so you cannot say that he uses or doesn't uses speedhack. You need to provide a demo if you ask me.

Regards,
Popay