Your question is unclear as to whether the informant is being used as a witness, tipster or source of information for a search warrant. In general it sounds like the informant was used as a source of information for an officer who was the affiant in a search warrant. If this is the case then usually the officer/affiant spells out in some detalil in the affidavit for the search warrant why that officer believes the affiant to be trustworthy; this is usually done by either having corroborated much of the information provided by the informant, or by the officer/affiant explaining how he/she has used this informant in the past and that the informant's information has always proved reliable. If this is the situation that you are confronted with than usually you do not have the right to question the confidential informant, (CI). Your attorney can challenge the sufficieny of the probable cause in the affidavit for the search warrant, however, the informant can only be questioned if the judge were to rule that the affidavit was legally insufficient on its' face.
The law recognizes that there are times where an individual may be willing to report criminal activity, but may wish that their identity be withheld. Generally the police attempt to assist such citizens (some may prefer to call them "snitches") so that such citizens 1) stay alive, and 2) help out again in the future. In fact, in some cases, the confidential informants may actually be paid by the police.
Having said that, there may be instances where the defendant's constitutional confrontation rights outweigh the informant's desire to remain in the shadows. This may be the case when the informant not only observed the offense, he or she actually participated in it.
The police are always allowed to be anywhere that any non-police citizen may lawfully be. So if a crime can be detected by the officer by simply driving or walking up to a specified public place (street, parking lot, park, club, back alley, etc.) and the officer literally sees the entire crime, the informant's identity is irrelevant. The officer can testify to the entire crime, and the fact that an informer led them to the crime makes no difference.
Similarly, if a citizen reports a drunk driver, an officer may arrest the driver if the officer sees a traffic violation with his or her own eyes, even if the informant anonymously brings the driver to the officer's attention. If the officer does not observe a violation, however, a stop based solely upon an anonymous call may be ruled illegal.
The identity of confidential informants are generally protected. The short and simple answer is: If the information provided by the CI is the only evidence the State has, then the State must provide the ID of the CI. However, if the CI provides information that is used only to obtain probable cause and further evidence is found after arrest or execution of a search warrant, then the ID should not be disclosed.
For example, a CI makes several purchases from a known drug location. Officers obtain a search warrant and find drugs at the location. The CI should not be disclosed because the officers have found further evidence of the crime and can prove the case without the CI. Further, the law protects the ID of the CI in this situation, so that the police can use him/her in the future without fear of the CI being identified.
However, if the CI makes several purchases from a known drug location and a subsequent search warrant produces no evidence, then the State must reveal the CI's ID and produce that person at trial or other pretrial motions.