Criminal intent must be formed before the act, and it must unite with the act. It need not exist for any given length of time before the act; the intent and the act can be as instantaneous as simultaneous or successive thoughts.
A jury may be permitted to infer criminal intent from facts that would lead a reasonable person to believe that it existed. For example, the intent to commit Burglary may be inferred from the accused's possession of tools for picking locks.