Root rot is fairly unlikely in modern systems that use live organisms to keep the roots protected. Signs are usually leaves sagging, or the lowest leaves beginning to yellow. Plants steal nutrients from their oldest/lowest leaves when they need food and the plants roots cannot supply the food. This is usually a symptom of root rot or root damage.
Prevent root rot by maintaining a good air to nutrient ratio - not wet and not dry but moist - and use a product with root microorganisms like Drive
If we have root rot, just watering less is not always going to fix it, and if it does, the plant may take a while to recover. If it is from overwatering, allow media to dry out a bit, and then feed a little less or less often. Use a sanitant like Hydroshield to clean the system, then start adding organisms like Drive to start root development and protection all over again.
If the root rot has come from cuttings from a sick plant, you will need to use a systemic fungicide like rot stop, rotguard and so on as the infection is on the inside. Hydrosheild will do a better job on the outside of roots and in the media and water.
Contact us for more specific advice for your growing method/system so we can tailor the solution for your situation.