The guilty mind is simply the intent or knowledge of an offense. In some cases, reckless behavior can impart enough of a guilty mind to satisfy the requirement. A great example of this is homicide by vehicle while DUI. Driving while intoxicated is a voluntary act and all the state would need to prove is that the individual had knowledge of their impairment. However, the homicide component would normally require some level of mental intent. In the case of a homicide by vehicle while DUI, the requisite intent is garnered from the inherent negligence of the DUI. The guilty act is the action itself in violation of a law.
The fact that the intent and the act must be contemporaneous is likely more beneficial to the defense than the prosecution. The ability to prove intent is sometimes a complicated matter. In most cases, for the authorities to launch an investigation, the act has occurred and is known. It is the peek into the mind of the offender to glean intent that makes prosecution more difficult.