Complaints are visible as speech bubbles over a prisoners head. Depending on the severity of the need the speech bubble will be round or jagged. The most severe need is displayed as an icon in the bubble. For more information about the specific needs of a prisoner hover over them with the cursor and table will pop up.
Watching out for these icons is important since they indicate which needs are not met, which can result in unrest, destruction, a riot or an escape attempt. Complaints can vary from food and bladder to safety and freedom.
You can see an overview of all your prisoners' needs by opening the Needs tab. Reduce any high (orange) or critical (red) needs to reduce complaints.