By sending Captain Kirk, of course.
Curiously, Starfleet seems to be the only law enforcement organization operating in the Federation (apart from a single agent from some sort of a "secret service" called Federation Security in ST3:TSfS - a man who subsequently sent his suspect to a Starfleet gaol). If enforcement is so monobloc, it wouldn't be difficult to speculate that laws are as well, and that only a single locality creates laws in the UFP.
Of course, such speculation runs into the problem of Kirk not really knowing the laws he's supposed to enforce. In "Amok Time", he actually gets himself killed because he doesn't know Vulcan law regarding duels to death! In "Cloud Minders", he encounters a long-running enslavement practice on the Federation planet Ardana, a practice apparently well known to others in the Federation (as they are longterm clients of Ardana on the mineral zenite), a practice openly admitted to by the local government in the same phrase where they threaten to complain to Kirk's superiors about his interference as is their right as UFP members.
Picard at least typically stumbles with alien law in cases where said aliens hope to become UFP members in the near future. And Sisko and Janeway operate outside the Federation, leaving us somewhat starved of useful evidence, as Kirk lived too early to tackle holographic simulation laws.