You and your former spouse have joint custody of your children. He or she has always been a good parent, so you had no reason to challenge your co-parent's request for shared custody in the divorce.
Lately, however, you have reason to believe that your ex has an alcohol or drug problem that could be putting your children in danger. Maybe your kids innocently mentioned something. Perhaps you noticed that he or she was "off" the last time you exchanged the kids. Maybe mutual friends mentioned that your former spouse had been drinking a lot lately or you found out that he or she got a DUI.
Your primary obligation is to keep your kids safe. If you believe that your co-parent has a substance abuse problem that could put your kids at risk, you can and should take action. You and your attorney can seek a child custody hearing.
Judges won't just take a parent at his or her word. Sadly, too many parents falsely accuse their exes of substance abuse issues to get sole custody. Judges will generally order an investigation to determine whether the parent has a substance abuse issue that puts the kids at risk. It's also helpful for the parent who goes to court to have evidence. That evidence may not be as obvious as police or arrest reports. Your attorney can advise about providing evidence to corroborate your case.
While the investigation is being completed, a judge may require that the parent under investigation have only supervised visitation with the kids. If a parent is ultimately determined to have an issue that could make him or her engage in reckless behavior around the kids or neglect to properly care for them, that supervised visitation will likely continue until the parent completes a recovery program and/or otherwise demonstrates the ability to care for the kids alone.
If you suspect that your co-parent has a substance abuse issue or is engaging in other behavior that puts your kids at risk, the first thing you should do is consult with your attorney. He or she can advise you of the best course of action and guide you through the legal steps you need to take to protect your children.