Answer:
YES! In many cases a criminal informant is working off some type of case or charge by helping law enforcement officials acquire evidence for an arrest and a conviction. One way to challenge the reliability of the informant is to expose what type of activity they were previously involved in and how desperate they are to work off that case by helping the law enforcement officials. This doesn't always work but it does help disprove the actual reliability of the informants statements. Talk it over with an attorney first, this is not legal advise just a simple attempt to answer this question.