Disclosure of an informant's identity may be required if the informant was an eyewitness to or participated in an alleged offense. Ford, 179 S.W.3d at 210 (citing Anderson v. State, 817 S.W.2d 69, 72 (Tex. Crim. App. 1991)). Similarly, disclosure may be required if the informant was present at the time of the offense or arrest or was otherwise shown to be a material witness to the transaction.

When it is shown that an informant was an eyewitness to an offense, that informant can in fact give testimony necessary to the fair determination of guilt or innocence. Anderson v. State, 817 S.W.2d 69, 72 (Tex.Crim.App. 1991). During the hearing on the motion to disclose, Appellant called Deputy Gutierrez to the stand who testified a Detective Guzman had informed him that a particular vehicle would be loaded with narcotics in the area, and it was based on this information that Deputy Gutierrez surveyed the area for the described vehicle in order to discover a traffic infraction that would enable him to pull it over. Appellant then called Detective Guzman, who testified that a confidential informant had provided his office with the information regarding the vehicle.

For example, "[w]henever it is shown that an informant was an eyewitness to an alleged offense[,] then certainly that informant can in fact give testimony 'necessary to a fair determination of the issues of guilt, innocence.'" Anderson v. State, 817 S.W.2d 69, 72 (Tex. Crim. App. 1991). Additionally, the State may not refuse to disclose an informant's identity if "information from an informer is relied upon to establish the legality of the means by which evidence was obtained" and "the court is not satisfied that the information was received from an informer reasonably believed to be reliable or credible."

Under Rule 508, a defendant bears the burden of making a plausible showing that the informant could give testimony necessary to a fair determination of his guilt or innocence. Anderson v. State, 817 S.W.2d 69, 72 (Tex.Crim.App. 1991). "The informer's potential testimony must significantly aid the defendant and mere conjecture or supposition about possible relevancy is insufficient."

An appellate court does not determine harmlessness by examining whether the evidence overwhelmingly supports a verdict of guilty; rather, the court should calculate the error's probable impact on the jury in light of the existence of the other evidence. Anderson v. State, 817 S.W.2d 69, 72 (Tex.Crim.App. 1991); Harris, 790 S.W.2d at 587-88. To reach such a determination, the appellate court should first isolate the error and all its effects and then ask whether a rational trier of fact might have reached a different result but for the error and its effects.