What is the test for determining whether a witness who testified for the government is an "interested party" in a trial?

The judge charged generally, as well he might, that the jury should take into account the interest, if any, which a witness had in the outcome of the trial in evaluating the testimony of such witness and that the appellant, who testified in his own defense, was an interested witness. Reagan v. United States 157 U.S. 301, 15 S.Ct. 610, 39 L.Ed. 709. At the close of the charge the appellant requested the court to charge that the narcotics agent who testified for the government "is an interested party in the same sense that the defendant is in a similar degree". Regardless of any lack of timeliness in making this request, it was such an obvious exaggeration of the interest of the agent that there was no error in refusing to give it.