There are many ways, usually by impeaching his credibility or character. However, it really is fact dependent and case-specific. Talk to the attorney handling the case.
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All of these steps require an attorney. Get an attorney - they need to obtain from the Superior Court the application for the Search Warrant. Then, assuming the attorney knows what issues to look for in a search warrant application, they need to go through and look for errors in written or verbal application. Good luck.
There are ways to do this but it is difficult and complicated and you will need an attorney to even have a shot at accomplishing this task. These are very fact specfic issues and each case and procedure of law enforcement is very important so you wil need an experienced attorney to check out what happened, how, when, etc to make sure it was done correctly and argue against anything that was incorrect. Good Luck and get an attorney now.
Controlled drug purchases present real challenges for a criminal defendant and for defense counsel. Usually the police will testify that they have searched the informant both before and after the purchase, and that they monitored the purchase while it occurred. But if there is anything vague or ambiguous in the way that the informant worked, or there is a gap in the police surveillance, the defense can exploit it. In this situation, where the informant participated in the sale, his/her identity must be disclosed to the defense. The informant's criminal history has to be produced by the prosecution, and there may be other evidence that can be used to impeach the informant. An experienced attorney who handles criminal cases, especially drug cases, can probably help you with investigating the informant's background by filing motions and researching the informant's history.